Nobody. Literally nobody emerged strong enough as an alternative to Romney to be a credible next in line figure in 2016. Everyone who was the anti-Romney essentially was just that, not a credible person to be the next in line.
Nobody from 2012 will probably be a serious contender in 2016. The simple reason is that the 2012 field was what it was because the GOP was decimated in 2006 and 2008, thus taking out top contenders who could have run in 2012.
I should note that the Democrats now have this problem.
That is a fair point.
McCain might not have won the nomination in 2008 if his opposition included Bill Frist and George Allen.
However, the list of plausible nominees who lost those cycles does seem kinda small. Aside from those two, who was there? Maybe Robert Ehlrich of Maryland.