IMO:
Halo 4 > Halo 1 > Halo Reach > Halo 3 > Halo ODST > Halo 2 > Halo Wars
Halo 4 was the best looking and most innovative.
Halo 1 is a timeless classic.
Halo Reach is a great sub-story action game.
Halo 3 was THE reason to get a 360 during 2007.
ODST just felt like Halo 3.1, mehhh.
Halo 2's campaign was a nightmare, boring and monochromatic.
Halo Wars, I just didn't like at all.