Even if Clinton does run in 2016 someone else will win the nomination.
I actually highly doubt that. I'm starting to agree with what Joementum wrote about a year ago, which is that if Clinton runs for the nomination in 2016, she might end up clearing the field, and perhaps receive only token opposition. (A la Al Gore in 2000, where he only had one challenger of any significance, and he ends up winning the primaries in all 50 states.)
In contrast to the situation back in 2006-2008, when there were serious doubts about her electability in a general election, Clinton is now the most popular (or nearly the most popular) politician in the country. Assuming she doesn't do anything to screw that up in her last year as SoS, she'll probably still be uber-popular in 2015, plus there'll be this residual sense of "unfinished business" about electing the first female president among a large segment of Democratic primary voters. Given that, I highly doubt anyone in the Democratic Party establishment will put up a fight against her. Whatever primary challenge she would face would be from the Left, but Clinton would be heavily favored to win that fight.
That said, the answer to this question is still Romney, because he has about a ~35% or so chance of winning this November, and even if Clinton would have an easy time at winning the 2016 Democratic nomination, it's a tossup as to whether she runs in the first place, and a tossup as to whether she wins the general election.
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