I think Gears of War is absolutely fantastic, but I'm going to have to go with Halo. Simply because I cannot remember a game with as amazing an opening as Halo had. Escaping the Pillar of Autumn, to me, signified that we'd reached the "next gen". So, in truth, there may be a nostalgic component there, but I don't consider it much of an insult to say "Not quite as good as Halo". That's like being "Not quite as large as Jupiter".