Chromehounds.
It's the best damn mech game I've ever played. The day they shut down the servers was the saddest day my Xbox ever experienced.
I mean, really. It had everything (except a good single-player experience). You could customize everything about your mech (and I don't mean pick a cockpit, left gun, right gun, and booster of your choice- I mean pick the legs/wheels to use then put any part anywhere you want on top. The customizing alone was amazingly fun), then play in a game that was more than just "run in and shoot everything." You had to work as a team, coordinate, cover each other, and stay in constant communication. (Which might be why it failed. It had a decent sized dedicated fanbase, but most xbox live users just want to kill with no waiting and scream at people, so most just ignored it.)