My 2 cents - the two opponents I've enjoyed the most playing against were The Dictator and DrunkDestroyer. I won against the 1st and lost against the 2nd.
The Dictator was really a great, fun and polite guy, regardless of whether he was winning 2-0 or losing 2-3 at the last moment. It's pretty easy being nice when you're crushing the opposition, but it impresses me when someone goes through a frustating adversary comeback, being 1 hp short of winning on game 5, and can still praise the game and be extremely courteous.
DrunkDestroyer was all around a great guy to play against, very polite and very nice to chat with during and after our match. And he's rooting for

in the final rounds to boot.
