Halo 3 easily for me.
Most important campaign mode obviously. Good storytelling. Fantastic multiplayer. Ahead of it's time for 2007. Great package.
Close 2nd for the underrated Halo Reach. I think Reach has the best campaign mode/storytelling although not the most important overall for the series. Gets a lot of undeserved criticism.
Halo 4 is honestly a pile of garbage. ODST was decent.








