I love Halo 3 a lot, probably one of my top games easily, everything about it is classic. Also I spent way too much time practicing it years ago so I'm mega biased just because I'm pretty good at it. Shame they fucked it up in the Master Chief Collection though.
Gears of War 1 was great too, if only the netcode wasn't total shit but aside from that the gibs were the best of any game I've ever played. The music was great and the shotgun frenzy multiplayer was brilliant. I always played with all weapons swapped to Torque Bows and Frags since it made for really intense games. I don't think there's another game with the same amount of fun as Execution was done, it's easy to not give a shit but when you're the last guy standing it's awesome that you can still clutch against a full team.
Forza 2 because I think it was Gran Turismo 4 but better. No licence tests, driving that didn't suck, nice looking cars and the customisation was my favourite bit since I liked NFS: Underground a lot for that.
Need for Speed Most Wanted 2005 is also excellent, only thing I wish they didn't change between it and Underground 2 was that they removed being able to fishtail opponents.
Only other games I can think of off the top of my head right now are Doom, Quake 1 and Quake 3. I don't play that much anymore though.
I loved MK3 and UMK3 for 2 player but that input reading scumbag AI put me off UMK3 on my own. :/