If you enjoyed Halo:CE and/or Halo2 then I would say get Halo3. I personally have always really enjoyed the Halo trilogy, I would say that Halo3's campaign and storyline were more interesting then Halo:CE's and certainly a much better game then Halo2. I don't think you will be disapointed if you get Halo3.
If you are looking for a good sandbox shoot everything/everyone you see then I would suggest getting GTA4. The GTA series has always been good fun for when you don't want to think or do some linear missions.
If ou enjoyed shooting Locusts with your Lancer then definately get GoW2. I found the campaign to be much more immersive and simply a lot more fun. One of the best games that I have purchased for the 360.
Having played all of these games, and many many many others, I would recommend getting GoW2.