I think Walker's political career is over. The strongest candidates are probably the youngest ones, so Rubio, Cruz, Paul. I wish Paul the best but the Republican base just isn't there for him (meaning they don't like his libertarian-ish style), and that is a fact that is very depressing for a person like me. And good luck to Cruz after that trainwreck of a primary last time. If he ever becomes the nominee (unlikely), I could see Trump not endorsing him as "payback" for him not endorsing Trump during the Republican convention, and that will only hurt and divide the party.
Marco Rubio reminds me a lot of Richard Nixon. That's all I really have to say about him. He'll probably be President some day and I would bet good money that he has, at the very least, some very shady connections.
Those connections were the Bushes, he gave those connections up when he went against them. Nixon was connected to Eisenhower's old friends. Anyway, it is now in vogue to bash Cruz due to the tight race with Beto, but people are ignoring that both Scott & Nelson have higher approvals than Rubio, who didn't run this cycle.