Doctor Strange
Abilities: Doctor Strange is a Master of the Mystic Arts, a sorcerer sworn to defend the fabric of reality. He has near-infinite stores of magic and can bend aspects of space and time (even though he can't use the Time Stone anymore).
Other Masters have noted that Strange seems born for the Mystic Arts and, according to the Ancient One, Strange is destined to be "the best of us."
Power scale: 8.5. The nature of the Mystic Arts does put Strange at a slight disadvantage in "Infinity War" because he needs to move his arms and hands to access his power, which is how Ebony Maw was able to capture him. But we can't hold Strange's loss to Thanos against him because he knew how that battle would turn out. He knew the Avengers had to lose first in order to win. So we don't know how Strange would have fared in that fight if he hadn't seen his own death occur in advance — but we do know that without him, Tony and the Guardians wouldn't have stood a chance.
In "Endgame," Strange mostly focused on practical damage control, so we didn't see him directly face off with Thanos again. We did see that he can teleport full armies from across the galaxy though, so that's a huge plus.