None of them is good for pot legalization with the possible exception of Ron Paul, who has about as much chance of winning the nomination as I do. The religious fundamentalists will never let Paul be the GOP nominee b/c he is a libertarian. Fundies want your behavior to be dictated by their definition of morality and not by individual choice.
The GOP will once again pander to their fundamentalist base. However, because the GOP is traditionally seen as the "tough on crime party," things probably won't get much worse than they are now because Republicans don't have as much to prove. The possibility for DEA reform and cannabis decriminalization will be nil, though, and most importantly, the justice department is already stacked with extremely right-wing, fundamentalist lawyers from religious law schools, so there will be no significant movement toward rational laws for cannabis under a Republican.
On the other hand, the left, as represented by Obama, Clinton, and Edwards, wants to control citizen behavior because they are statists who believe in the supremacy of the state over its citizens.
Any individual autonomy is a threat to a statist. All three of the Dem contenders will toughen up on drugs just to look tough on "crime." Remember, it was Bill Clinton who
executed a retarded guy in '92 to look tough for the primaries. Expect Obama to be toughest of all three Dems because of his schtick about using drugs in high school. He'll have something to prove. Hillary is too terrifying to consider -- whatever the polls indicate is the media-driven perception of drugs will dictate her policies. She's just as likely to impose harsh fed prison time for simple possession as she is to begin decriminalization, depending on her approval rating and on focus polling at the time she is expected to act -- the only thing worse than a politician with bad principles is one completely devoid of any principles whatsoever. Finally, Edwards has no chance -- the guy is an empty suit, so I wouldn't expect him to be much different from Hillary.
They are all bad choices because they are all politicians. Don't vote -- it only encourages them.