Chromehounds 2
The first was amazing and underrated. Or rather, it was a PC game that was only released for the 360, which was a bad mistake. I mean, it's the only 360 game I played from the day I got it to the day the servers shut down without it getting old. 'Twas a slow paced mech game that was an amazing alternative to any of those fast paced laser mech games that dominate the industry.
It was the only game that offered you that big of a degree of customization, and actually MADE you work as a team to get anything done. To do good, you had to design your own good hound beforehand, test it to make sure it works, then find a good squad and cooperate with them to take out the enemy. The result? A small devout following that played it till the day it went offline, and a huge number of people who hated it because they couldn't just hit "multiplayer" and jump straight into some fast paced shooting action. *sigh