Fallout3, hands down. It is a great game, involving strategy and free exploring. Although the game is somewhat timed, I believe, it still allows the player to interact with the enviroment. Also, it is one of only a few games left in this world that involves a realistic and sensible plot.
Fallout 3 is good coz its by Bethesda(creators of Elder Scrolls), Mercenaries is great for co-op/online, Command & Conquer is agreat RTS, but i dont know about it on Xbox, and grid, i have no idea what that is and Soulcalibour is just lie another streets fighter, or tekken just all characters.. Hope that helps?