Depends on what you want in a game.
Since you haven't bothered to tell us what you're into, I'll just say that Halo 3 gives you much bang for your buck:
splitscreen, online splitscreen, co-op, and Forge - tons of replayability.
KZ2 was great at first, but the game feels like it lacks depth. I'd probably be playin it more if was as tactical as GG wanted it to be. Unfortunetely, they let a few certain classes ruin that part....