Who is the LEAST POWERFUL being that can defeat Thanos?
...Thanos himself. because Thanos is Thanos's worst nightmare......
IIRC he's pretty much gotten over that problem, but good choice.
Start with Squirrel Girl and work your way down
^^^^^^^^^^^ Because a issue wrote by Larry who doesnt even write comics, where Thanos get hit by cosmic cube, tried to rob a bank unlike what he has ever done before, and the story is mocked by the creator of Thanos(Jim Starlin) is a very good scan to use.
On a serious note, from DC likely Despero/ Doomsday could, from Marvel some of the fancy Thor villains like The Destroyer Armor could.