It's hard to pick really, Halo 1 introduced me to the franchise and made me fall in love with gaming in the first place. Halo 2 had the Arbiter and better fleshed out the multiplayer experience with the inclusion of the Elites. Halo 3 had the bonus of so much extra content and excitement built up around it. ODST was a fun and interesting new approach to the experience and Halo: Reach was, well, Halo: Reach.
I can't really pick a clear winner here.
I can't really pick a clear winner here.