I'd have to say COD4.
While the single player was a little short, it was, without a doubt, the most kickass single player campaign I've played in a while. There isn't much replay value to the single player (you can re-do it with cheats etc.), but that's where the multilayer comes in with plenty of ranks, challenges, and unlockables.
Next would be a Crysis / Bioshock tie.
Bioshock was a very unique experience. The game had a fantastic story-line and you actually cared to learn about the characters. (Unlike some other games where a main character dies and you hardly give a damn - probably can't remember their name either. 😛) After you finish the campaign once through - that's it. You probably won't play it much after that.
Crysis had very innovative graphics and interactive (semi-destructible) environments. The campaign is so-so, not the best, but not the worst. You can kinda see how much more time was spent on making the game look good, rather than on the gameplay it self. Again, not a bad game, but certainly not the greatest. The "sandbox" design is much more free than Bioshock - so replaying a level can result in a different experience each time, so that's a plus. Multi player... eh, I really dislike it. Having no permanent rank / class selection makes the game a bit repetitive and tedious. (Buy weapons, vehicle, ammo, etc after each death.) :/