Romney has the best chances of getting the nomination. He has 2 things that help him get it 1)They've found the least amount of dirt on him (unlike other previous candidates). and 2) He's getting a lot of support from people like chris christie, which really makes a difference as the arch conservatives love chris christie. If christie actually becomes the vp with romney, i think obama will have a hard time in the next election.
All of this is unknown though. Anything could happen with politics.