I agree that Obama probably is the best bet of the current crop of mainstream candidates, but he's not going to touch that hot potato. Too controversial. Too much propaganda and misunderstanding out there. If you look at it from a standpoint of whether a Democrat or a Republican is more likely to make a decision like that, though, generally a trend toward change away from the status quo is much more likely to originate from a Democrat, most likely a liberal Democrat. (Obama's not exactly liberal, although arch-conservatives like to say he is.)

The legalization issue isn't something that's likely to be a decided on a national level anyway or with some sort of constitutional amendment, IMO. Most of the organized pro-cannabis movement, like NORML, for instance, knows that it'll be a gradual process that starts on the state level with a.) legalization for medical use, b.) decriminalization, and c.) legalization for recreational use.