No. 3 North Korea
Is Kim Jong-il alive? Or, the better question is, will the regime be even more menacing if he's incapacitated or dead?
Of all of the U.S.'s adversaries, none is more unpredictable than North Korea because there is just so little information available. Add to the fact that North Korea potentially has nuclear weapon capabilities, the danger posed by this rogue regime cannot be understated.
While Kim's fate is uncertain, the regime has not ceased its intransigent behavior that dated as far back as the Korean War. The latest is the regime's unilateral announcement that it's closing its border with South Korea. With South Korean president Lee Myung-bak taking a harder line, the North is perhaps spoiling for a new confrontation.
More ominously, the regime also declared that inspection of its nuclear program could not go beyond the Yongbyon facility and the inspectors could not leave with samples from Yongbyon for testing. That declaration was in breach of a deal brokered by Secretary of State Condoleezza Rice that resulted in North Korea's removal from the U.S. list of state sponsors of terrorism.
If Kim does not resurface, the regime apparently would be run by a military junta similar to the one that's in power in Burma - only more diabolical and perhaps with nukes to boot. Will they decide to simply carry on with their usual assortment of blackmail tactics, or set the world on fire?