3 was my least favorite of all the ones I played through. I don't even really have a strong opinion about Reach, because I stopped playing it. If Reach had come out back when 3 did, I'm sure I would have played all the way through that one--it's less a comment about the quality of the campaign, and more just that I was really into Halo back when 3 came out.
Looking back more objective, Halos 1 and ODST are my favorite campaigns. 2 is really maligned but it was more interesting than 3.
I'm not sure how I feel about 4 yet. I've completed the first three campaign levels on Legendary, and I'm out of town for Thanksgiving now so I can't make any more progress for a few days. I really dislike the skullface guys so far. I could be reading too much into this, but they seem like a strong example of an enemy type Bungie wouldn't have made. They just seeme very overcomplicated, without being really interesting to fight. All of the Covenant types have very strong identities that, when combined into different groups, make for a very complex potential combat scenario. But just one of those big skull guys feels like he takes a whole ton of head shots at range, and also can basically one-shot you up close, and also warps around, and also has those little flying shield guys--when I know I have to fight a bunch of those guys, I just already feel exasperated. Whereas when I run into one or two of the huge Covenant hulking beam guys with weak points on their back, I know they can totally wreck me if I'm not careful, but they don't have like five different gimmicks I have to keep in my mind.
Halo is about very simple but robust things--its complexity arises from the interesting and varied arrangements of those simple elements. The Flood in earlier Halo games bothered me for the opposite reasons the skull guys do--they were just ONLY simple elements, with none of the interesting complexity that arises from variety.
I'm sure part of it is that I'm playing on Legendary, and I haven't internalized the tactics to use against these guys in a safer environment, but it's still hard for me to imagine that the elements that make up the skull guys wouldn't make for a more interesting scenario if they were simply distributed out among multiple existing or new types of Covenant, rather than all jammed into these crazy super-units.