Rubio is the only one with a good chance to win. We know that Hillary doesn't connect well with younger voters. Bernie is kicking her ass in that regard. We also know from Nevada that she doesn't seem to be connecting with Latinos very well, because Bernie beat her with them, too. If a 75 year-old Jewish socialist can attract young and Latino voters away from Hillary, Marco can, too. Plus, even if they just don't vote, Marco benefits. Finally, right now Rubio polls 57-41% when matched up versus Hillary.
Rubio is clearly the best shot Republicans have to win, and it's not even close. The only hope (and it's not a small one, imo) that the Democrats/Hillary/Bernie have is if there are enough stupid people voting in Republican primaries and caucuses to install Trump (and to a much lesser degree Cruz) as the candidate. Cruz has a slight chance to win, Trump will get totally creamed, as the media will finally give equal time to both candidates.