Walker could beat Bernie Sanders, probably Martin O'Malley too. But I can't see him beating Hillary.
I've been arguing he'll be the nominee for awhile now. He has made some mistakes early on but who hasn't thus far? And unlike the rest of the field, he actually has a record to run on. Not just "I fought against Obama on x...and he still did x" posturing like the senators in this primary field. Walker's legislative record is a conservative's dream. He has cut taxes, destroyed unions, gutted public education, restricted voter rights, etc.
He doesn't greatly offend any of the GOP's three ideological pillars. I think it's pretty clear Bush's "moderate" game isn't going to work. The party won't let Paul come close to the nomination. Rubio could win but I think all the senators are at a disadvantage in this primary. Cruz will flame out, but not before taking others down with him.
Walker Rubio, count on it.