Q2 was amazing
Loved Doom, but never played it online. It was only when Quake II came along that I discovered the joys of killing an LPB (low ping b*stard) opponent with a railgun shot, despite my ping being 10x higher than his! Particularly liked the Lithium mod, with the hook to propel you quickly around the map. But my all time favourite for frantic gameplay was CTC (catch the chicken). Awesome, but particularly so when played using a custom Homer Simpson avatar, complete with sound effects like 'Woohoo!' "D'oh!" etc. Happy days.