Marathon wasn't PC, though. Maybe the last two, much later, but it was always primarily Mac. Right?
For me, I call System Shock 2, then Thief 2, then Deus Ex, then Half-Life. Tribes is probably one of the greatest multiplayer games ever, and Doom deserves props for the sheer number of hours I spent playing it over a LAN in high school. Ahh, those were the days...