I'm probably biased since I've played all of them but Mazy's spot on. Excellent AI, very challenging, great coop fun, awesome multiplayer, fun weapons, great story, design, etc. Halo has it all.
Plus, it didn't follow the shooter trend of becoming a on rail game with one scripted event and cutscene after the other while shooting hordes of retarded grunts in tight linear levels surrounded by invisible walls ![]()