Who Kills Thanos? - Predicting Avengers: Infinity War's Climax

2. Doctor Strange

In terms of pure power, there are few characters in the MCU who can rival Doctor Strange and the fact that he is the Sorceror Supreme and the universe's appointed protector suggests that he might be the most fitting candidate to take down Thanos.

By the time he has command of some of the Infinity Stones, Thanos will need someone with more than just brute strength and will to defeat him and Strange's magic could be the game-changer.

And even more crucially, Strange's command of the Time Stone means that he could be capable of undoing Thanos' acts and finding a means to defeat him in the past. That much has already been hinted at strongly for Avengers 4 (or at least that would explain a time travel element), and it really wouldn't be surprising if Strange - as the new leader of the Avengers - becomes the key to taking Thanos down.