christie and rubio are the only two legitimate contenders on the GOP side at the moment, but the eventual nominee is probably someone who's not currently in the conversation. yes, it always surprises people how quickly presidential candidates emerge, especially from the non-incumbant party: virtually NOBODY thought Obama had a shot at the 2008 nomination back in 2004, for example. even the biggest Obama boosters (the ones who were blown away by his '04 DNC speech) were thinking 2012 or 2016 at the earliest.
we all assumed Romney was the front runner for the GOP nomination from more or less 2008 on, and he ended up getting it, but that's actually a bit unusual. it's to the GOP's shame that he was the only even remotely credible contender they could come up with in 4 years. normally, the guy that catches on is someone that's a little bit of an up-and-comer and enough of an unknown factor that people can fantasize a bit -- project what they want onto what he has shown, and not have there be too much of a disconnect. Romney had all kinds of problems in that department. too much of a record. too many contradictions.
which brings up an interesting point: i think history (or at least recent history) shows that serious political experience is generally a detrement to Presidential aspirations, for better or worse. compare the relative political experience levels of obama, gw bush, clinton, and reagan to their opponents and i think it's a bit lopsided in favor of the losers of those elections!