Favorite 2-player games

What are your favorite 2-player video games? (Something that can be played on either two PCs or one XBOX/PS3/any other system)

Preferably co-op. Competitive is okay but it drastically reduces the chances of Chris and I playing it because competition between us usually devolves into one of us being pissed off at the other one :P

Some games we have played together co-op:
Gears of War (1 and 2)
Resident Evil 5
Borderlands
Eternal Sonata (this one is not so great co-op)
World of Warcraft
Warcraft III
Diablo II
Final Fantasy Crystal Chronicles
Rock Band
Super Mario Bros. Wii

I especially loved playing Gears of War, Diablo II and Resident Evil 5 together. (Borderlands was fun, but not as fun for some reason. I kind of hated the Borderlands split-screen, and I thought RE5 and Gears of War used the split screen a lot better) Well, and World of Warcraft, but I don't want to play another MMO.

I do love Super Mario Bros. Wii but I prefer it with more than 2 players, same w/ Rock Band :)