Well, Halo: Combat Evolved.
Best level design
Best physics
Best offline multiplayer
Best sound design
Best story
Best weapons
Best animations
Best damage/health model
2 bollocksed up everything (especially the sound, physics, weapons and animation) whilst 3 fails epically at level design and the graphics looked no better in motion than Halo 2. 2 and 3 were far too reliant on memorisation and 1 hit kill enemies to add difficulty, not to mention the awful, awful flood levels (seriously, did anyone play test them?). After eight years Halo:CE still plays best and is the most fun, so it wins.