IDK if this counts but The Two Towers is my personal favorite
The first movie lacked the epic battle sequences, there was no Gollum, and Frodo didn't struggle at all with the weight of the ring.
In general, I'm not all that big on sequels and prequels - not because I'm opposed to the idea but because they rarely live up to my expectations.
One group that does is the "Before" series from director Richard Linklater with Ethan Hawke & Julie Delpy.
They go in this order, and I really can't say which I like best because I think they're all great in their own way.
I would argue that most sequels are produced entirely from greed.
I am limiting the term to movies that were truly single stories. Not
the "Star Trek" sequels or comic book characters (they were already
part on an ongoing series of adventures) and not the Lord of the
Rings movies (they were based on books).
I respect those who count them, I just don't.
An example is "Aliens". There was no need to continue O'Bannon's
story beyond that monster on a ship story. Yet Cameron did, and
did it well.
"Back to the Future" fits. A stand alone story with no need for a
sequel. I know I'm in the (vast) minority but part II is not needed and
not a good movie.
Anyway... far too much info. But I do think that "Empire" is a better
movie that "Star Wars".
Surprising that this is a hard one to answer. Surprising that so many are so bad. (I don't know why i imagined the new Beetlejuice would be good, but I was kind of appalled by how un-good it was). But anyway. Of course, yea yea yea, Godfather. Beyond that i can think of Bride of Frankenstein, and maybe Aliens.