I was disappointed with Halo 3 [Halo 1 is still my favorite of the series], but then again, I don't care much for multiplayer games; I only do the campaign.
Gears of War was awesome, but too short. They need to make a sequel with twice as many levels in it.
Assassin's Creed I really enjoyed. Some complained that the investigation missions were too repetitive, but it didn't feel like it to me. I hope in the sequels they keep the realistic look of the cities while adding some depth to the play.
Trying to get into Half-Life 2, but it's an older game ported to XBOX, and doesn't stack up to the games listed above.
