(CNN) -- Any compilation of the world's most dangerous terrorists is a hazardous undertaking, a shifting list that's open to endless debate.
If you live in Moscow, Chechen Islamist leader Doku Umarov would feature prominently. Many Israelis would likely include Hezbollah leader Hassan Nasrallah on their list and people living in the southern Philippines, the Abu Sayyaf group.
Some terror figures who were among the most wanted several years ago, such as Abu Anas al Libi -- who was captured last weekend in Libya -- appear not to have been active for some time. Even some terrorists try to retire. The last list compiled by CNN included senior al Qaeda operative Saif al Adel. He has vanished from the radar and may have been under house arrest in Iran.
Other figures lose relevance as their group loses territory, membership and/or funding. Groups such as al Qaeda in the Islamic Maghreb have been prone to internal rifts. Additionally, al Qaeda, especially in Pakistan, has moved away from identifying senior operational figures because of the effects of U.S. drone strikes, so some of a new generation of most dangerous terrorist figures may not yet be known to us.
The following selection is intended neither as definitive nor a "league table." It focuses not on organizations but on men (and they are all men) alleged to be plotting, directing -- and in some instances carrying out -- acts of terror aimed at causing mass casualties among civilians.
Some are ideologues and planners, others "operational," and some are both. They think and act in a regional and in some cases a global context. Some of the individuals below have appeared on previous lists compiled by CNN and others, and have lived long enough to warrant a second or third appearance.
Others are only now making a name for themselves among the world's counterterrorism agencies, as they take advantage of conflict or the collapse of state authority, forge new alliances or develop new ways of bringing terror to the international stage.
Despite the whittling away by drone attacks of "al Qaeda central" in the mountainous border region between Afghanistan and Pakistan, the group's leader remains vocal and active in trying to harness the disparate affiliates that claim the al Qaeda name.
Since former leader Osama bin Laden's death in 2011, al-Zawahiri has sought to take advantage of the unrest sweeping the Arab world, and has recognized that groups such as al Qaeda in the Arabian Peninsula (AQAP) and al Qaeda in the Islamic Maghreb are better placed to carry out attacks than the ever-diminishing core that remains in "Af-Pak." At times, al-Zawahiri has struggled to exercise authority over groups such as the Islamic State in Iraq, not least because of the difficulty in communicating with far-flung offshoots.
Aware that pulling off another 9/11 is a remote possibility, al-Zawahiri has suggested a shift to less ambitious and less expensive but highly disruptive attacks on "soft" targets, as well as hostage-taking. In an audio message in August he recommended taking "the citizens of the countries that are participating in the invasion of Muslim countries as hostages."
Al-Zawahiri, an Egyptian doctor who is now 62, is not the inspirational figure to jihadists that bin Laden was, but he is trying to fashion a role as the CEO of a sprawling enterprise. According to the Economist, he may be succeeding. "From Somalia to Syria, al-Qaeda franchises and jihadist fellow travellers now control more territory, and can call on more fighters, than at any time since Osama bin Laden created the organisation 25 years ago," it wrote this month.
Reward offered by the U.S. government for his capture: up to $25 million
2. Nasir al Wuhayshi
For someone thought to be about 36 years old, Wuhayshi's terror resume is already extensive. Once bin Laden's private secretary in Afghanistan, he returned to his native Yemen and ended up in jail. But not for long: He and several other al Qaeda operatives dug their way out in 2006. He went on to to help found al Qaeda in Yemen, and began launching attacks on Yemeni security services and foreign tourists, as well as directing an ambitious attack against the U.S. Embassy in Yemen.
He is now the emir of AQAP, widely regarded as the most dangerous and active of al Qaeda's many offshoots. A slight figure with an impish sense of humor, according to some who have met him, Wuhayshi appears to have been anointed al Qaeda's overall deputy leader in a bold move by al-Zawahiri to leverage the capabilities of AQAP. Seth Jones, a Rand Corporation analyst, called the appointment "unprecedented because he's living in Yemen, he's not living in Pakistan."
If al-Zawahiri is al Qaeda's CEO, Wuhayshi appears to be its COO -- with responsibilities that extend far beyond Yemen. It appears that in 2012 he was already giving operational advice to al Qaeda's affiliate in North Africa.
Despite a concerted effort by the Yemeni government and the United States to behead AQAP, Wuhayshi survives, and his fighters have recently gone on the offensive again in southern Yemen. The group is bent on exporting terror to the West -- both through bomb plots and by dispatching Western converts home to sow carnage.
3. Ibrahim al-Asiri
Not a household name, but one that provokes plenty of anxiety among Western intelligence agencies. Al-Asiri, a 31-year-old Saudi, is AQAP's master bomb-maker, as expert as he is ruthless. He is widely thought to have designed the "underwear" bomb that nearly brought down a U.S. airliner over Detroit on Christmas Day 2009, as well as the ingenious printer bombs sent as freight from Sanaa, Yemen, and destined for the United States before being intercepted thanks to a Saudi tip-off. The bombs were so well hidden that at first British police were unable to find one device even after isolating the printer.
Al-Asiri also fitted his younger brother Abduillah with a bomb hidden in his rectum in an effort to kill Saudi Arabia's counter-terrorism chief, Mohammed bin Nayef. The brother died in the attack; bin Nayef survived.
His trademark explosive is PETN -- a white, odorless powder than cannot be detected by most X-ray machines.
Al-Asiri is thought to be somewhere in the vast mountainous interior of southern Yemen. The anxiety among Saudi and Western intelligence officials is that he has passed on his expertise to apprentices.
4. Ahmed Abdi Godane
Godane, aka Mukhtar Abu Zubayr, became the leader of the Somali group Al-Shabaab at the end of 2008. Traditionally, Al-Shabaab has been focused on bringing Islamic rule to Somalia, and as such has attracted dozens of ethnic Somalis (and a few Western coverts) from the United States and Europe. But Godane appears to be refocusing the group on terrorist attacks beyond Somalia, against the east African states that are supporting the Somali government -- especially Uganda and Kenya -- and against Western interests in east Africa.
The Westgate Mall attack in Nairobi September 21 was Al-Shabaab's most audacious, but not its first nor most deadly outside Somalia. In 2010, Al-Shabaab carried out suicide bombings in the Ugandan capital, Kampala, in which more than 70 people were killed. But the Westgate siege, which left 67 people dead, demonstrated Godane's desire to align his group more closely with al Qaeda. In a taped message afterward, he noted the attack took place "just 10 days after the anniversary date of the blessed 9/11 operations."
Under Godane, Al-Shabaab has become a formal ally of al Qaeda. That has led to dissent, which Godane has dealt with ruthlessly, using his control of Al-Shabaab's intelligence wing. The American jihadist Omar Hammami was killed in September after criticizing Godane's leadership and his treatment of foreign fighters.
Godane is said to be 36 years old, and is originally from Somaliland in northern Somalia. He is slim to the point of wispy, as seen in the very few photographs of him, and prefers recording audio messages to appearing in public.
After the Westgate attack, Kenyan and Western intelligence agencies will undoubtedly step up efforts to end his reign of terror. But he should not be underestimated. A former Somali prime minister, Omar Abdirashid Ali Sharmarke, once described Godane as the cleverest of Al-Shabaab's leaders.
The U.S. government's Rewards for Justice program lists him under another alias, Ahmed Abdi Aw-Mohamed, and is offering up to $7 million for information leading to his location.
Belmoktar is Algerian but based in the endless expanse of desert known as the Sahel. Like many on this list, he has an uncanny knack for survival against the odds. A year ago, he probably would not have been counted among the world's most dangerous terrorists. Then he announced the formation of an elite unit called "Those Who Sign With Blood," which he said would be the shield against the "invading enemy." A short time later, his fighters launched an attack on the In Amenas gas plant in southern Algeria. A three-day siege left nearly 40 foreign workers dead.
Since then, Belmoktar's fighters have launched attacks on a military academy and French uranium mine in Niger in May, despite losing much of their freedom of movement after the French intervention in Mali in January.
Belmoktar is unusual in combining jihadist credentials with a lucrative business in smuggling and kidnapping. He is often called "Mr. Marlboro" because of his illicit cigarette trafficking, and is thought to have amassed millions of dollars through ransoms for westerners kidnapped in Mali.
Intelligence officials have told CNN that he has also developed contacts with jihadist groups in Libya as instability has gripped the country in the wake of Moammar Gadhafi's overthrow.
Born in 1972, Belmoktar grew up in poverty in southern Algeria. He traveled to Afghanistan in 1991 in his late teens to fight its then-Communist government, and returned to Algeria as a hardened fighter with a new nickname "Belaouar" -- the "one-eyed" -- after a battlefield injury. He later joined forces with the Armed Islamic Group (GIA) in its brutal campaign against the Algerian regime.
Reward offered by the U.S. government: up to $5 million for information leading to his location.
6. Abu Muhammad al-Julani
While Belmoktar might have been on the fringes of a "most dangerous terrorist list" a year ago, Abu Muhammad al-Julani would not have been anywhere near it. But as Syria has descended into a state of civil war, al-Julani's group -- the al-Nusra Front -- has emerged as one of the most effective rebel factions. Formed in January 2012, it is a jihadist group with perhaps 10,000 fighters, many of them battle-hardened in Iraq. It has specialized in suicide bombings and IED attacks against regime forces, and its success has attracted hundreds of fighters from other rebel groups.
Al-Julani personally pledged his group's allegiance to al-Zawahiri in April, and the U.S. State Department has branded al-Nusra as part of the al Qaeda-affiliated Islamic State in Iraq. In May, the United States added al Julani to to the list of Specially Designated Global Terrorists.
Al-Nusra has so far not shown any inclination to take the fight to Western targets. Andrew Parker, the head of the British intelligence agency MI5, thinks that will change.
"A growing proportion of our casework now has some link to Syria... Al-Nusra and other extremist Sunni groups there aligned with al Qaeda aspire to attack Western countries," he said in a speech in London this week.
Of al-Julani himself, very little is known. Al-Nusra places a premium on organizational security. Even his nationality is unclear, but he is thought to have had experience as an insurgent in Iraq. A recent study by the Quilliam Foundation in London concluded his leadership of the group was "uncontested."
"Sources tell us that his face is always covered in meetings, even with other leaders. Al-Julani is thought to be a Syrian jihadist with suspected close ties to Abu Musab al-Zarqawi and al Qaeda in Iraq," the study's authors said.
Al-Zarqawi was killed in a U.S. missile strike in 2006.
7. Abu Bakr al-Baghdadi
One factor that may influence the growth and potency of al-Nusra is its relationship with fellow jihadists in Iraq. Abu Bakr al-Baghdadi, the leader of the Islamic State in Iraq and Syria (ISIS) was publicly at odds with al Julani over the regional pecking order earlier this year, asserting that al-Nusra was part of his group, a claim swiftly rejected by al Julani. Western intelligence would like nothing more than dissent between these two groups. Close cooperation between them across the long Syrian-Iraqi border -- the goal of al-Zawahiri -- is the nightmare scenario.
On the battlefield in Syria, cooperation between the two groups appears to be continuing, especially in towns like Deir Izzor in eastern Syria.
Inside Iraq, al-Baghdadi has overseen a dramatic spike in terror attacks against the Shia-dominated state and security apparatus, aided by jail breaks and bank robberies. It has also claimed devastating bomb attacks against Shia civilians and is open about carrying out attacks on purely sectarian grounds. It claimed credit for a wave of car bombings in Baghdad on September 30, in which more than 50 people were killed, calling it a "new page in the series of destructive blows" against Shiite areas in Iraq.
The monthly number of civilian deaths in Iraq, according to the United Nations, is now at its highest since 2008.
Al-Baghdadi benefits from fertile ground in that Iraq's Sunni minority is increasingly fearful of the Shia-dominated government led by Prime Minister Nuri al-Maliki. Sunni tribes straddle the Syrian-Iraqi border, adding to a combustible regional picture.
Born in Samarra, al-Baghdadi is in his early 40s. In a eulogy for bin Laden, he threatened violent retribution for his killing. Analysts regard ISIS as a greater threat now than at any time since the U.S. "surge" and the emergence of the Sunni Awakening Councils six years ago, which then turned the tide against al Qaeda in Iraq.
Reward offered by U.S. government, which lists him as Abu Du'a: up to $10 million for information leading to his location.
Shifting from the Middle East to the Afghan-Pakistan border regions, several groups are positioning themselves for the exit of U.S. combat forces from Afghanistan next year. Among the most dangerous is the Haqqani Network, responsible for some of the deadly attacks in Kabul in recent years. A 2008 coordinated suicide bomb attack on the Serena Hotel in Kabul left six dead. Another strike in June 2011 killed 12 at the InterContinental Hotel.
U.S. officials say that in addition to its high-profile suicide attacks against hotels and other civilian targets in the Afghan capital, it is responsible for killing and wounding more than 1,000 U.S. soldiers in Afghanistan.
Known as Siraj, Haqqani is the son of the group's founder, and is in his early 40s.
"Siraj is a brutal criminal murderer," Gen. Jeffrey Schloesser, the outgoing commander of the U.S. 101st Airborne Division in eastern Afghanistan, told the publication Jane's in 2009.
Jeffrey Dressler, a senior analyst with the Institute for the Study of War, told CNN last year that Haqqani is "very, very competent, a very capable leader who has really grown the network over the past five, six years."
U.S. officials say the Haqqani Network is all the more dangerous in that its presence in the tribal territories of Pakistan is tolerated by the Pakistani government. The family belongs to the Zadran tribe, which spans the Afghanistan-Pakistan border and stretches to Khost province. The Haqqanis have a close relationship with both al Qaeda and the Taliban, but are also thought to have begun recruiting Chechen and Turkish jihadists.
The Obama administration designated the Haqqani Network a terror group last year. It is regarded as well-funded because of a series of legitimate and illicit businesses that stretch to the Gulf.
Reward offered by U.S. government for information leading to Haqqani's location: up to $5 million
9. Abubakar Shekau
Shekau's inclusion recognizes the growing tide of Islamist militancy in West Africa. For the last four years, he has led Boko Haram, a Salafist group in northern Nigeria that has begun cooperating with other groups as far away as Mali.
But its main focus remains churches and other Christian targets, the police and the moderate Muslim establishment in northern Nigeria. Just last month, suspected Boko Haram fighters broke into a college in Yobe state and murdered more than 40 students as they slept.
In 2010, Shekau warned that the group would attack Western interests and the following year it carried out its first suicide bombing -- against U.N. offices in the capital, Abuja -- killing at least 23 people. The group has also kidnapped and killed several Western hostages. While Bokko Haram is not an affiliate of al Qaeda, Shekau has made clear his sympathy for the group's goals. The United States made him a Specially Designated Global Terrorist in June 2012.
Two caveats here: there are conflicting reports that Shekau was killed in an August raid by Nigerian special forces. But a video that appeared weeks later purported to show he was still alive. And Boko Haram's leadership structure is opaque at best; it's unclear how much control Shekau himself exerts over its fighters.
John Campbell, a former U.S. ambassador to Nigeria, wrote last month that so far "Boko Haram has shown little interest in the world outside of Nigeria and the Sahel. But the situation in Nigeria is dynamic, and it is possible that closer ties will develop between al-Qaeda and elements of Boko Haram."
"Boko Haram" means "Western education is forbidden" and reflects the group's utter rejection of modernity and Western influences.
"Hostile to democracy, modern science, and Western education as non-Islamic, it is highly diffuse," Campbell said of the group. "For some adherents, religious, even apocalyptic, themes appear to be paramount."
Reward offered by the U.S. government: up to $7 million for his location.
10. Doku Umarov
Doku Umarov leads the Caucasus Emirate (CE), a Chechen group dedicated to bringing Islamic rule to much of southern Russia.
The U.S. State Department named Umarov a Specially Designated Global Terrorist in 2010, and said subsequently he was "encouraging followers to commit violent acts against CE's declared enemies, which include the United States as well as Israel, Russia, and the United Kingdom."
U.S. officials have been investigating whether the Tsarnaev brothers -- who were blamed for carrying out the bombing at the Boston Marathon in April -- had any links with Chechen militant groups. But nothing has surfaced connecting them with CE. And the group's main focus has been on attacking Russian institutions and civilian targets. In January 2011, it bombed Moscow's Domodedovo airport, killing 36 people, and suicide bombings of Moscow subway stations in 2010 killed 40 people.
Umarov was born in southern Chechnya in 1964, according to Chechen websites, and describes his family as part of the "intelligentsia." He came of age as the separatist campaign against Russian rule began to take root and joined the insurgency when then-Russian leader Boris Yeltsin sent troops into the region in 1994.
In a proclamation published on a Chechen jihadist website in 2007, he declared, "It was my destiny to lead the Jihad... I will lead and organize Jihad according to the understanding, given to me by Allah."
Reward offered by the U.S. government for information on his location: up to $5 million.