I thought Halo 2 had a really fun campaign but maybe that is just my sweet memories playing tricks on me. I loved the arbiter missions. The multiplayer was fun, but I had only played that with my brothers (again nostalgia).
Halo 3 had the best multiplayer for sure, the maps were really fun, the vehicles were fun to use. Equipment was actually a really good idea, they should have kept that for Reach as well. The campaign was equally fun to play minus the cortana mission and the gravemind/cortana slowmo breaks. I actually went back and played multiplayer in like 2013 and it was fantastic.
I would give Halo 3 the best Halo game award.
On that note, I felt Reach was bad. The campaign wasn't really fun, I can hardly remember it. The multiplayer just wasn't the same, it felt off. The loadouts were a bad idea, they should have kept the equipment, and not added abilities to cater to the COD fans.
I actually hated ODST, played it through once, was not impressed.
Halo 4 was very fun! The campaign that is. The multiplayer can go die. I mean the gameplay felt very good, it was just satisfying, the soundtrack was well done. It had a couple tracks that were very memorable. The mission where you have to fly through the gauntlet was very well done, much better than the shitty one they had in reach. The protheans were not to bad to fight, much better than fighting the flood in any case.