There are 2 games that really stand out for me:
"Gears Of War" was just amazing. When the original came out it's all I played for absolutely ages. Excellent multiplayer.
"Chromehounds" is probably the greatest Team Multiplayer ever made... ever. In the universe. The diversity of the different mechs and the ways different types supported each other as a Team was phenomenal. Greatest tragedy in gaming when those servers shut down.
What about you guys? What's your best MP experience on XBox and why?