I enjoyed Halo 1 for the coop aspect primarily, but hardly played multiplayer. I was anticipating that game ever since it was a Mac exclusive and my dad showed me a trailer of it (huge Mac fan) in high school.
I got to play Halo 2 early from a friend who lent me a modded Xbox with the leaked French version. It was enjoyable and probably added the most to the series from all the sequels, but again, I played no multiplayer.
I have barely touched Halo 3, and none of ODST, which I may like the most based on the coop aspects.
I watched the trailer for Reach and it looks interesting to say the least. It will probably sell very well, but I’m hoping it will have something interesting to add to the series. It will at least give some background to the entire mythos but prequels don’t always fare well. Not to mention that there is always an issue adding new content to a prequel as it creates continuity problems (add a new enemy and people will ask why it wasn’t around for Halo 1/2/3), e.g.). I’ll check it out but want to know more, especially since I’ve yet to buy a Halo game.