Assassin's Creed is a beautiful game, and the lead character really blends into the world - you really feel like you're climbing, and that you have weight. The game itself, well, to be honest I got bored with it midway through and sold it. Very pretty, though.
Uncharted has some gorgeous scenery, but the Drake character always felt disconnected from it, especially when climbing steps and such. It was hard not to notice.
Burnout Paradise is very solid, and has an amazing sense of depth - lift your eyes away from the car and check out the backgrounds as you're cruising around, it's almost 3D in feel.
COD4 is the same old shit with a prettier skin.
Gears of War is sharp, but oh so generic. And hey, let's wear heavy armor everywhere but our heads, nice design decision dumbasses. ROLLEYES
I love the deco background art style of Bioshock, but the enemies are all samey after a while.
I'd give it to Ass Creed for now. Very immersive.