I personally like 3 better. But as Phase said, it was the last Halo by Bungie and it's seen by many as the last truly great one.
Also, from a more anecdotal point of view, it was the last time Halo felt like a big deal. I was working retail when it launched and there was this big excitement for it, with big lines, a midnight release with official cosplayers, MS sending us tons of merchandise, etc. MS even sent personalized dogtags for everyone in the store were I worked at with our gamertags printed on them.
Then with 4 excitement went down. I still remember it selling well and all but it was treated as more of a regular AAA release as opposed to this big event