Accepting that the field consists solely of Trump, Rubio and Cruz, Rubio has the best change of uniting the party and winning the general election (avoiding a Sanders or Clinton presidency). Winning an election means holding on to Republicans and winning independent and Democratic voters, and Rubio is in the best position to do that. Rubio started out as a “Tea Party favorite” defeating the establishment Republicans in Florida, though hard core nativists will oppose anyone who isn’t a pure blood WASP with roots in the 17th century.
The caveat is that Trump can back down on many of his positions that help in the primaries but not in the general election (e.g. nativism, which as it is sounds weirding coming from someone whose ancestors came through Ellis Island and who is married to an immigrant). While Trump’s views on most issues clearly qualify him as a “RINO” (a.k.a. a Rockefeller or Eisenhower type Republican), conservatives will see him as the lesser of two evils when running against. Trump is running on “anger” rather than platform, so he can adjust his platform to have broader appeal, but unless he does so he is very unlikely to win the election – meaning we could get a vehemently anti-religious socialists as president.