Note: This is all based on campaign and multiplayer and all of this is mearly opinion.
Halo C.E. - Great campaign but lacked a fun and exciting multiplayer.
Halo 2 - Had a nice new campaign experience by giving us two different ways to see the Halo universe (Master Chief and the Arbiter), but it was still fairly short and not as great as Halo C.E.'s campaign. It had a meh-ish multiplayer experience but was the beginning of Xbox Live for Halo and I applaud Bungie for that.
Halo 3 - I believe that Bungie's main focus on Halo 3 was it's multiplayer because they gave us brand new gametypes and a variety of different armors for both Spartan and Elite character models (and much more), but its campaign was short and not very exciting which dissapointed me and many others.
Halo Wars - This was definitaly a new and different experience for most Halo fans. We were all used to the classic FPS feel that Halo had but Ensemble Studios decided to change it up into a more strategic game. Halo Wars's campaign was short for the most part but had a cool feature of turning on/ off A.I.'s for a custom match which was nice practice. I had a lot of trouble getting used to the feel of it, though.
Halo 3: ODST - Bungie's latest game. I'm sure many people were dissapointed on how short the campaign was and how Firefight lacked a matchmaking feature, but besides that it had some nice things to it. For starters, the flash-backs were cool, it gave a new campaign experience, and Firefight was something I never expected to be on Halo, it is pretty exciting even when you play it out solo.
I hope Reach will have a great mixture of both campaign and multiplayer, not just focus on one, or focus on both but make them terrible and boring.