Halo 1 had my favorite campaign, as right away you're given a definite threat to fight. The Chief has already been fighting the Covenant for years, so there's no reason to try an understand them, or think that they might be misunderstood. You just know to kill them. However, you get the fear of the unknown in the form of the Flood, and later when you discover the true purpose of the Halo ring. Also, as much as I don't love "The Library" I do enjoy the adrenaline rush that it gives me. (Still need to get "That Just Happened"...)
I spent so many weekends having Halo 2 bashes with my buddies from High School that I know all of the multiplayer maps like the back of my hand. Campaign wise, it introduced my favorite character of the series, the Arbiter. I've always liked characters that have a heel-face-turn role in stories. Overall, I just enjoyed the familiar yet still alien feel of the game.
Halo 3 seemed a little short campaign wise, but I enjoyed it nonetheless. The biggest plus for me was the addition of Spartan customization.
ODST was fun for a couple of playthroughs, but nothing special.
I'm not a huge fan of RTSs so I didn't play Wars.
Reach for me was about like ODST.
I particularly like 4's multiplayer, but I'm finding the campaign to be a bit rushed.
Halo 2 had the most disappointing campaign IMO. It was a rushed mess, but at least the multiplayer was amazing. In terms of multiplayer Reach disappointed me the most. It was so slow paced and majority of the maps were terrible.
Posting from my smartphone, I'm trolling from the future!