I've seen people play Diablo 1 and 2 but never played either myself. Don't know why I never got around to it either, I'd probably like the games. Sim City though, that was one of the first PC games I've ever played, at a summer computer camp where we played on black and white Macs. That, Wolf 3D, and Comanche (remember that one?)... I don't normally like the sandbox god game genre, but there's something about the Sim City games.
Big series I've never played:
-Baldur's Gate series (this needs to be rectified, I'd really like to try 2)
-Ratchet and Clank series (I might get the PS3 one though)
-Any Tom Clancy game (no interest whatsoever)
-Diablo
-Command & Conquer
-Any MMO game... (no interest)