Nightfire was awesome, used to play multiplayer all the time. Loved that ski resort map. Resident Evil 4 is my go to answer, but I also gotta mention Smash Bros Melee, some of the wrestling games were good that generation, Paper Mario TTYD. Soul Calibur 2. and THPS 4.
That was always fun. I miss the "fun" guns in multiplayer games. Stuff like the sentinel or laptop gun in perfect dark. Every multiplayer game prioritizes balance over fun it seems.