Ted Cruz -- in his first term as a Senator. It would be fun to watch the righties who screamed that Obama wasn't qualified turn around support him. Not surprising, but fun.
Rubio's for real. I'd be very scared of a candidate like Rubio if I were the Dems. Jeb Bush will probably give it a go, but the question is does anyone really want Bush III? Scott Walker might, but I'd be shocked if he's able to get the nod -- Too much anti-labor baggage.
The real deals for the GOP didn't run in '12. They didn't want to blow their wad going against an incumbent. They'll be there for '16 though.