I think Marco Rubio is now the front runner for the GOP nomination, and apparently the betting markets think so too.
NYT I think Clinton's Benghazi appearance and experience (and political machine) will easily win her the Democratic nomination (sorry, Bernie). And I think Trump and Carson will flame out once voters start getting serious. Rubio has the backing of GOP establishment donors, now, as JEB!'s star is fading.
When it comes to brass tacks, though, the electorate will go with expertise over youth. Rubio is telegenic, youthful and well-spoken (he is a lawyer, after all), but he is callow, has spending skeletons in his closet, has little political experience and will have immigration issues to walk back (that his base will not like). Clinton will keep him on the defensive, and he'll never gain traction.