I'm sure this question get's asked all the time, but what are the best two player games for Xbox? Any suggestions or favorites?

Well that depends.
If you for instance are into sports titles in single player mode, chances are you're gonna like them even more in multi player.
Both Fight Night Round 2 (albeit not for all tastes), Pro Evolution soccer 4 and Tiger Woods 2005 comes higly recommendeble.

Also, Burnout 3 is a hoot in multiplayer...instantly accessible and just so much fun.

Freedom Fighters
Halo 1 & 2
Certainly, Tiger Woods 2005
Burnout 3
Splinter Cell Chaos Theory has a really enjoyable co-op.
Madden 2005
Ncaa Football 2005
Tony Hawk's Underground
MX Unleashed

Just about any sports game. I have great fun with all of ESPN's 2k5 releases as well as madden 2005. Tiger woods is a pretty good one too. Halo and Halo2 are great additions and are some of the best coop games I've ever played.