Time Splitters 2 or MOH: Frontline (yes, an EA game that's actually really good) for FPS.
I rate Mario Party 4 also. In fact, my mum and sister are playing MP4 right as we speak (they've gotten bored of Mario Party 8 after playing it for 1.5 years!).
Double Dash is pretty fun also. Way more fun than than MK Wii.
There's still fun to be had on Monkey Ball I & II.
Desert Storm had its moments 4 players (and some depth).
Phantasy Star Online.