Halo 4 is the best campaign in the Halo franchise.
Story is incredible, Master Chief is more badass, more heroic than I've ever seen him, he is more human, he has more depth, this is the version of Master chief you read about in Halo: First Strike by Eric Nylund. Cortana's portrayal, the relationship with the Chief, the way it expertly involved the lore from the books and extended universe, the Didact as an enemy, the unbelievable production values and incredible gameplay pacing. I don't think anything in the franchise tops it.
It rescued the Halo franchise as far as I'm concerned, which was growing stale, and I felt was in trouble after Halo 3, ODST and Reach, and to this day it remains the bar to be topped by any future effort. A lot of people pretend to be Halo fans now, but I've read this forum and seen the same comments crapping on Halo when even Bungie were on it. It wasn't really until 343 came onboard that all of a sudden, to attack the new Halo studio and to crap on the Halo franchise, that suddenly the rose colored lenses were adopted regarding what people were saying about most recent Halo games. None of the Halo games were terrible games, but they were to me severely falling off in delivering on the potential of their story universe, and just didn't deliver that excitement I felt was warranted of Halo. I was hyped before release of every Halo title, and enjoyed them in my own way, but after the fact I just felt like it needed to be better. Halo 4 delivered that. Halo 5 was good, I enjoyed it, but the story had a lot of unmet potential, it felt abrupt, like it was a transition before something else bigger. I liked a lot of the ideas it tried though, especially bringing in Blue Team, and I don't have any issues at all with Fireteam Osiris. Wasn't quite as epic as the hunt the truth podcasts though admittedly.