Sup Legged :)
Personally my favorite is 3, not that I can really judge having only played 3, ODST and Reach. Halo 3 was the most fun for me because it was the most alien. It made me wanted to explore, they were huge open spaces and huge structures, which appeal to me.
Although, having said this, that isn't to say I didn't enjoy ODST or Reach. I loved the detective and mystery style of ODST and the urban fighting that the setting provided. I also enjoyed the ending of Reach as it was a great example of combining story and game play, you kept fighting until you got over run, and then came the inevitable cut scene.