I haven't done research for the non-mainstream parties yet, but for Democrats, I'd pick Mike Gravel followed by John Edwards. I don't really like any of the Republican candidates right now; they're too lackluster. And McCain is a turncoat, totally un-trustworthy - he'd easily turn into a Richard Nixon.
And I absolutely hate Hillary and Obama. Most like Hillary because she's a woman, and they like Obama because he's black. Neither is worthy of the presidency at all - the first black president needs to be so over-accomplished no one can object to his initiation, as with the first female president.
Gravel reminds me a lot of Truman - small town, fiery, and with a lot of potential to do great things. Most of all, he's NOT AFRAID of the public, nor of making himself look bad. Like Truman, he doesn't have a ghost of a chance, but if he were to get into office things would be different. He's more liberal than I would like (and from Alaska, no less) but I doubt he'd pursue as many agenda issues as he's put stances on.
I feel sort of bad that I'm not some rich well-to-do fellow, elsewise I'd donate to his campaign. :(