Quake (1). Single player was great with the NIN soundtrack, but the multiplayer is what did it for me. CTF, DM, Arena, TF, and all the crazy mods that came with them (runes was good). From Netquake (lavakings server), to Quakeworld (Madhouse server), it didn't get better than Quake (1).
-Aeurix
-Aeurix