Who Kills Thanos? - Predicting Avengers: Infinity War's Climax
9. Hulk
Although their first meeting is likely to end badly for Bruce Banner, it seems that what he sees in Thanos at least impresses the Hulk within him (or that's the hint in the recent story detail reveals from EW).
Some rumours have suggested that we might even see Hulk side with Thanos temporarily (either through coercion or choice after Banner suppresses him), but there's still a big chance that Hulk might be the one to take Thanos down. In terms of pure hardware, he's definitely the best equipped and he's the closest the current Avengers have to an invulnerable character.
Hulk's powers also determine that the harder he's hit, the angrier he becomes and the stronger he gets. So unless he's clicked out of existence, he's going to be a significant threat to even a fully powered Thanos.