We know Mitt Romney thinks it's Russia and now the White House is on record giving al-Qaeda the dubious honor, but neither of these answers seems all that satisfying. Romney's answer, redolent of the Cold War, at least has the benefit of anointing a bona-fide geopolitical heavyweight. The White House's response has the benefit of identifying a group that is actually implacably hostile to the U.S., even if its power is negligible.
So who should get the top spot? China, like Russia, has geopolitical clout but isn't hostile to the U.S. across the board in the manner of an al-Qaeda. Beyond China, countries like Iran or North Korea (or even Pakistan) could earn a nod for their hostility to U.S. regional aims, but again, not for their power or geopolitical weight.
Even conducting this thought experiment usefully illustrates the fact that the U.S. is actually in a pretty nice geopolitical position in 2012: it has very few implacable enemies and none that are very powerful. There are very powerful states that, on certain issues, play a spoiler role, but the era of straight-up great power antagonism is gone. As James Joyner pointed out, the entire notion of the U.S. having a "number one geopolitical foe" is an "outmoded concept."
At least for now.