Depends what you're wanting it for. From my point of view, I would say Mercs 2 is the best for single player, I've seen the PS2 version and I can assure you the difference in the 360 version is worth playing it again for.
I don't play online so when I say multiplayer I'm referring to playing with mates; I've had, and still have, a lot of fun with Gears 2. It really is spectacular. The style of play is right up my alley in that it's 3rd person rather than 1st, it's fantastically gory and is great fun with mates, though one player is a bit dull (I imagine playing online is great). Personally I love the horde mode (wave after wave of attacks). Halo 3 is a great game, but I think to get full pleasure out of it you need to play online, and it's one of those where I imagine the matches are full of 12 year old kids.