If the republican primaries were reasonable, I would say Ben Sasse. Whoever can be the most obnoxious will probably win, so I'll go with Rubio.
First time I may actually agree with a socialist on anything. But no Rubio finished he's never going to run for president again and if he does he has no chance of being the nominee. He probably wouldn't even be in the top 5 or 6 presidential candidates
To me, it seemed like, after his failed bid last year, Rubio immediately pivoted to running for president again. If Clinton had won last year, then Rubio would be running in 2020. But since he's unlikely to challenge an incumbent GOP president, he'll wait until 2024.
I think Rubio went into 2016 prepared for the possibility that he would lose the nomination, but live to fight another day, and would then run for president again in 2020 or 2024. He's young enough that he'll probably run for president again at least one more time, if not two.