I think Batman: Arkham Asylum is really up there and if you think not, here is a shining example (a little spoiler this way comes): fighting a Titan induced Joker for the very first time is just... epic. Half Life 2 is pretty heavy in the epic department, but for me, Call of Duty 4 takes the cake. It's like an action movie. Seriously. The first level (not counting the training stage) is already one of the most epic things in gaming history and combine that with an epic story line, epic good guys, epic bad guys, epic levels, epic guns and a truly epic ending, well, I'd say you've got yourself an epic game.