COD4 lacks in the multiplayer department. If you have a group of friends over only one can play online yet its campaign is superb which makes up for it! Whereas Halo 3's campaign IMO is awful! far to short yet the multiplayer makes up for it as it is a great laugh with or without some friends.

Best game ever on the 360 so far? hmm thats a difficult one I wouldnt be able to put my finger on it as all games have there pros an cons so I would say there isn't one that stands above them all.
once i've completed the campaign on either of the games or any game in that matter, I wouldn't really want to play the campaign again because I had already completed it. Thats where Multiplayer for me comes in.. Both COD4 and Halo 3 have awesome multuplayer but due to the fact that you can play with friends on the same console on Halo 3 and not COD 4 is why I choose Halo 3 over COD 4..

I'm sure many people will disagree/agree but this is purely my own opinion.

If I were to choose over Halo 3 or COD 4 I'd choose Halo purely on the fact that