CoD4 or Halo 3. I just can't say which I enjoed more.
CoD4 had a really good campaign and a sensible story (unlike MW2). Moving to the multiplayer, it has brilliant ranking system. The weapons and perks are mostly well ballanced (apart from M16 and Juggernaught) and you end up loving some of them like some crazy weapon person. The sound of the silenced UZI, headshoting with the M40 sniper, C4ing people to death and slaughtering enemies with the Desert Eagle that I can honestly call the most fun gun to use in any game are all unforgetable weapons.
Halo 3 on the other hand had a solid singleplayer, with an ok plot that didn't really live up to my expectations. What I did like was 4 player co-op and a huge ammount of hidden content all over. The multiplayer on the other hand is amazing. The matchmaking system works, even though it tends to be a bit slow and I've met some of my best online friends in Halo 3 matchmaking. But the best part of the game is playing user created content in custom games. The insane amount of creativity of some of the maps are crazy. Running down a maze while being chased by a fat kid, playing cops and robbers, hunting people down in a recreation of the movie predator and playing demolition derby are some of the most unforgetable multiplayer experiences I've ever had. And the theater mode is a great feature too.