Greatest Question Ever! Who is the strongest character in fiction?

George Orr.

(From "The Lathe of Heaven" by Ursula Leguin")

I guess both could hit a stalemate and the winner of the Dr. Manhattan vs George Orr would be the first person to be the quickest one to think the other out of existence.

There's not many to choose from, Mary Sues aren't that popular, there's not really anywhere to go with them.

In the Captain Marvel comics Thanos becomes a god. (I've not seen any of the Avengers films.)

Dr Manhattan may have been super powerful but he wasn't really that aware. He never twigged what happened. Thanos had his finger on the button and drive. He'll kill Manhattan when he's preoccupied with volcanic eruptions on Venus.