I already answered here...
John McCain if there's any possibility of that happening, though I see it as unlikely!!!
http://www.able2know.com/forums/viewtopic.php?t=16708&highlight=
The reasons are listed in the link above but the main ones are that he's a war hero and will get a LOT of moderate votes.
Clark doesn't seem like he has a lot of substance behind the man. He has no policy plans, all he EVER talks about is his experiences in the army. It sounds like he's running for Secretary of Defense, not for the Presidency.
Hillary Clinton is an even worse choice IMO. I just don't see anything she has done that suggests she's the best choice for vice president.