I dont think anyone (except christie or jeb or maybe some senior repub statesman like those old governors) could beat hilary.
Rubio wouldn't beat hilary - nostalgic swing voters that chose bill clinton, bush2, and obama will choose her.
Rubio is just not ready, IMHO - he needs more time. MAYBE if the dems run a softball field, he should jump in. But if i was rubio, I'd keep quiet, build an amazing record, and let the candidates beg me for the veep slot. Better to be freakin amazing at VEEP than barely prepared for Prez job. And hilary is gonna be a beast - probably one term, no holds barred, her last political charge - no newbie repub wants that kind of heat. Imagine bill clinton going 'all out'... brutal, dude.