If it is Trump vs Hillary, he might lose, and I do mean might. A LOT of disappointed democrats will stay home giving him the edge, but a lot of republicans will also stay home. It will come down to how hated Hillary is. The independents might not vote for either.
If it's Trump vs Sanders then Trump wins no problem. Few Americans except the radical left will vote a socialist into office.
Rubio would beat both DNC candidates. Therefor I would have to disagree that Trump is the best bet.
Think about this as part of a dynamic process if you really want to anticipate what will eventually happen.
Right now Trump is going up in polls, and there is no reason to believe he has some fictional cap. People were saying that about Romney in 2012 as well and were proven wrong. Trump isnt polling high as a second choice, but he has been a lot of peoples third or forth choice behind idealistic candidates like Carson and Kasich.
Hillary is dropping in the polls as she shows the world she is a cheating cantankerous old hag, cheating Sanders, lying to authorities about her email and secured document handling and may be indicted just prior to the election. Her poll numbers will continue to drop.
Rubio is a provenliar, and though he has rising numbers, as people become more informed with his track record he will start hemorraging supporters.
Rubio is a loser no matter who yo put up against him. He even makes Hillary look honest.