Bioshock.
Amazing game. From the moment I started I knew I was in for a ride. I popped it in, and I played seven hours straight. I missed two meals and didn't have time to shower before heading out for dinner that evening. Now, I have it for the PC, but they sell it for the 360, so i figured I would write it up.
The graphics are stunning, particularly the water effects. (Which is a good thing, considering the game is played in a submerged city) The makers did a fantastic job setting the mood using music and the decor. It's creepy sneaking around halls of a run down casino plaza, old record music playing from some hidden player. All around you is the evidence of a recently fought battle, trash cans still rattling down the stairs from the quick escape of the survivors.
The weapons in the game are the standard FPS types, Shotty, pistol, machine gun, rocket, wrench/crowbar, sniper/crossbow. But what stands out is the most used weapon is actually your own body. Through Plasmids, you gain powers such as fire from your fingertips, lightening, freezing blasts, and a whole bunch of others. It's a real treat freezing the oncoming mob in place with a few freezing attacks, then unloading your machine gun on them, shattering them into an innumerable amount of ice cubes.
I highly recommend this game, but I don't think it is good as Half life 2, which I thought I read it was coming out for the 360 sometime. Maybe not, i don't remember.
Anyway, buy this game.