PaddyPower, the Irish betting website. (The Florida senator is at far right in the picture above.)
Rounding out the top five are:
2. Chris Christie, far left (9/2)
3. Rand Paul, third from left (8/1)
4. Jeb Bush, not pictured (8/1)
5. Paul Ryan, second from left (9/1)
Texas Senator Ted Cruz is a distant 6th, at 16/1 odds.
Now I suspect that Jeb Bush doesn't have the fire in the belly for a White House run and that Paul Ryan is more comfortable in the House. So if Chris Christie is damaged goods, which I believe he is, that leaves Rand Paul and Marco Rubio to duke it out for the support of both the tea party and establishment wings of the GOP. And some of Paul's views on foreign policy and social issues are just too far out of the Republican mainstream. Advantage: Rubio.
P. S. My dark horse candidate: Gov. Scott Walker of Wisconsin (16/1) can also unite the two wings of the party.