It's probably:
1. Walker (and he will remain here until he says something stupid, because that's the only way the other Republicans will be able to go negative on him)
2. Rubio (Rubio will be the natural beneficiary when Jeb inevitably collapses and if/when Walker falters he will be the best positioned)
-Gap-
3. Cruz (He is by far the smartest debater/campaigner in the field, I have doubts he can win, but he is the definitive wild card in the race. I predict that even if he doesn't win he will have several 'kills' notched on his belt before the end of this)
Walker's made tons of missteps and he isn't even a candidate yet. He isn't ready for prime time. He won't be the nominee. Bush or Rubio. My money's on Bush.
What Republicans (typically speaking) fail to realize is that there is a moderate wing of their party. It's not just the overbearing establishment picking the candidate every cycle. Typically, the field isn't filled with moderates (or at least not filled with moderates who have a chance), and so one moderate scoops up the moderate vote, and the conservatives split the vote. I am confident that Jeb will be the nominee.
The only thing that is keeping Jeb Bush afloat is tons of money and name recognition, all of his support is soft and the GOP just isn't where Jeb Bush is anymore. If you want to talk about missteps, Jeb Bush has proven pretty clearly that his political instincts have deteriorated.