It depends on what you're looking for. Call of Duty 4 is incredible that has a short but great story mode, however the multiplayer in COD4 is excellent. There's a whole ranking/leveling up/achievement/unlock system that keeps you coming back to unlock more weapons and abilities. The only complaint I have about COD4 is that the players online can be annoying brats, more so than any other game I've played, and that's on the PS3 version where there aren't as many players and people with headsets. I can only imagine it being
FAR WORSE on Xbox Live. Thankfully no one in that game cares to cooperate as a team, so you can safely turn off the voice chat.

The Orange Box is one of my favorite games of last year. You get five kickass games for cheap. I think you should just get it for Portal alone, but that's only a four hour game at max unless you decide to play the advanced levels or go through the game with developer commentary. Half Life 2 and all the episodes are really good as well. From what I've heard, Team Fortress 2 on the 360 is not a very good port. I wouldn't be surprised becuase the PS3 version isn't all that great either. Feels like it would be much better on the PC, but it is still a good deal of fun on the console.
I've never played Assassin's Creed, so I can't offer much advice on that one. It does look pretty good though.
When it comes down COD4 vs Orange Box, I'd lean more towards The Orange Box since you get so much more bang for the buck. Either way you go you'll get an excellent game, but you just need to decide what kind of game you'd prefer.