Hillary Clinton, easiest question in the world. If Hillary decides not to run I'm going to laugh at all the Hillary hacks and the entire Democratic Party. Not to mention people act like she's got the election in the bag, as if the country can't possibly get more Republican, because you know, demographics and stuff.
Hilary Clinton is a Wallstreet Democrat, we have Martin O'Malley or Jim Webb or Joe Biden who will run despite a Clinton withdrawal who is less Wallstreeters. Hilary Clinton isn't a base Democrat, she is a consensus Democrat and just like if Harry Reid loses his seat, we will have a liberal democrat to replace Reid in Durbin or Schumer, and for president in 2016 with Martin O'Malley or Gillibrand in 2020.
But,
Jeb by far is the most overrated candidate, he is still losing to Clinton, and GOP donors want him instead of Romney