"Best" can mean everything and nothing, do you mean "who's the vilest?", "who's got the most fascinating character's arc?", "who's your favorite?"... the other problem is that I can't see any reading of the word 'best' that would make a viewer vote for Thanos, if you asked for favorite, Thanos could stand a choice...
... for me, the only fitting contender to Darth Vader is Michael Corleone.
