Definitely quake/quakeworld. Played that game for like 5 years, and to this day remains my favorite multiplayer shooter of all time.
It's too bad id abandoned their multiplayer roots...they were the first with MP modem play (doom), first with internet MP (quake), yet the studio that were the forefathers of multiplayer FPS games don't even bother with MP anymore. I miss the old id software.
I play quake live regularly almost everyday. I suck at it but sometimes manage to win some games.