For me it's Uncharted 3 without a doubt.
If it's not a fast and furious FPS game, then I need really immersive storytelling to stop the feeling that I'm simply looking down on the action. UC3 carries that off beautifully.
Dead Space is a different kettle of fish graphically and after a couple of hours play last night, it is already signposted story wise. That said I damn near shat my pants a couple of times around midnight. The sound in that game played via the surrond sound literally scared the wits out of me at times. It gets so intense, that even though you know mr mutant scissor your b****x has just popped out behind you, you still don't want to turn around! The graphics are not unpretty, but it's the sound that really makes this game shine. And I'm loving it.
Unless it does something silly later on, this is already my game of 2008. Better late than never.