John Kerry and Tim Pawlenty.
While Pawlenty running is right on the fringe of unrealistic, there's not a snowballs chance in hell of Kerry running again (just like Romney)
I'm going to say Hillary Clinton (not from a supporters standpoint... their a reason the PUMA's liked her, but when the press takes sides like in 2008, with Chris Matthews "Thrill up my leg" comments, than something is up) (Also by this point a voter who is paying attention should know just about everything there is to know about her. There's nothing new for her to tell or for voters to learn)
I'm also going to say Paul Ryan (Mostly for the same reasons as Hillary, plus the facts only Sheldon Cooper wannabes will actually vote for him, and those people typically aren't Republicans) and Marco Rubio (great optics, but not a good speaker at all)