Campaign : Halo 1. Big maps and long missions with much freedom to run around and explore.
Matchmaking : Halo 2. The first online multiplayer in halo. Beautiful maps, cool weapons, the best ranking system. Minus- & +pluss Glitchy but sometimes making it fun.
Custom games : Halo 3. Because of the custom games where you could play anything from normal infection to Jenga tower to Skycastle to Warthog wrestling. Minus- Hard to get into games with a lot of people due to no sort of custom game search option.
Forge: Halo reach. The ability to make almost anything and not using hours like in halo 3. Even though it felt more special with a good crafted map in halo 3 than on Reach, due to easy forge. Minus- Not so many maps to make it on(Forge world get a little boring after a while) and lack of pallets and movable objects in forge world.
Firefight : Halo 3:ODST. Amazing maps with strong enemies and not so powerfull characters(ODST) so you had to work as a team to survive. Minus- Having to play with friends because you dont have a search option.
Theater : Halo 3. Four friends going into theater to watch the match they just played, laughing at fails and ownage. pluss+ good way to check who had the best connection. Minus- Only four people at one time.
I didnt include Halo wars because i dont see it as one of the proper Halo games.
^ IMO ^__________