Many years ago, I had a discussion at university regarding the lack of movie sequels that were superior to the originals. At the time, there were really very few out there… Empire Strikes Back (quite contentious) and Godfather Part II (I’ve not seen it, but I’ve heard it’s better than the first) being the main two potentials. The other pair that often get brought up are Aliens and Terminator 2, which I’ll admit I enjoy more than the first movies, but that’s really a case of liking action more than horror; I’d have a hard time arguing they were necessarily better movies.
However, over the last ten years or so, there have been quite a number of superior sequels come out – X2, Spiderman 2, The Dark Knight, Iron Man 3 (we’ll gloss over the second). Now, this is not a case of bad movies with better sequels (in all cases the originals were excellent movies and that’s cheating anyway), but one thing you will immediately notice about all of these films is that they are all comic book movies; there is something about the comic book movie genre that seems to breed excellent sequels.
I think the reason that comic book movie fanchises fare so well in their second outings is that the origin story is out of the way. Assuming that a superhero is given an equally good treatment in his first and second films, the film that doesn’t have to spend the first third laying out groundwork is always going to be working at an advantage. This is also the reason that The Avengers worked so well; all the groundwork was laid in the individual films. Had they tried to produce The Avengers as a standalone, it would probably have been an absolute disaster… I hope the Justice League movie production team are taking notes.