Gus has the best chance, because all he has to do is beat Glover and he is in title talks, and he will fight for the belt after DC's next title fight.
Maia has the second best chance, because he has to beat Masvidal, but the UFC will probably wait for GSP vs Bisping to be decided.
Mousasi has the third best chance, but it depends on GSP vs Bisping. If GSP wins, the MW division will be in shambles, with no legitimacy. If Bisping wins, Mousasi will have to wait for an American fighter to win the belt, before the UFC will even consider giving him a title shot.
Barbosa is last on the list. Honestly, he will only get a title shot on short notice.