Call of Duty 4 is amazing. The singleplayer is a bit weak for a CoD game, but it's till very solid. The multiplayer is amazing, if you have a 360 you really need to buy CoD4, it's just ridiculously addictive.
Mass Effect is the game of the year. I've play Bioshock, Halo 3 and CoD4, all contenders for game of the year. Bioshock and Halo 3 are ridiculously overrated, they got great reviews because of all the hype, there was so much money spent on advertising. Anyway, Mass Effect is one of the most immersive Roleplaying games ever, it's not an RPG, it's a Roleplaying game.