Tetris DS, hands down. I always seem to get my arse handed to me whenever I play that game, though.
Also, I enjoy the occasional hours of online gameplay with Final Fantasy XI. I'm still somewhat a noob, but it's addicting, thats for sure.
Halo 2.
Halo 1 on PC!!! , But Once I get Windows Vista, I'll See How Halo 2 on PC is, It's Supposed to have a Few Extra Maps, Than the Xbox version, but IDK
Wow...talk about bringing a thread back from the dead >_>
To stay on topic before close: I used to play Ragnarok Online.