Halo 3 frustrated me, because I played on the same difficulty setting as the others but I died a lot more. Then again, I had the same experience with CoD4, but replayed it much later and loved it, so maybe I just need to play the game again. For now though, something has to be my least favorite of the series so I suppose that's it.
So, for me it goes:
1) Halo Reach
2) Halo 1
3) Halo 2
4) Halo ODST (which, really, is another game I think I might like more were I to replay it)
5) Halo 4
6) Halo 3
Altogether, with the possible exception of Halo 3, I don't think any of them are much better or worse than each other.
Oh, and I'm going purely off single-player, as I couldn't care less for multi-player.