Alan Wake
I got this for free with the XBox I got last Christmas, and never bothered to try it out until tonight. And so far, I'm loving it. The atmosphere during the night sequences is just perfect. The game was really creeping me out, and I'm only at the beginning (it doesn't help that it is dark and windy and rainy out, too).
I don't think I'm going to play this game in the day time, but hopefully tomorrow night I get a chance to continue. The game seems like it would be a much better experience at night.