Best Xbox game of all time?

This thread serves the same purpose as the original thread here: https://what-the-game.forumotion.co.uk/t107-best-pc-game-of-all-time but for games on any Xbox systems ever released, so if you don't have a clue what this is about: basically, here you can tell us your very favourite game for ANY of the Xbox consoles… and it has to be just one game and one game only - the very best if you had to decide.

I've never had an Xbox console, but I bet there are some here who will go for Halo and Gears.

I'd go for Halo 3, and you know I would Halo 3 is a non boring game, i love it when people say something like "can you help me on Halo 3?", i look for any excuse to play the campaign or multiplayer