I'm with y'all on these sequels:
The Godfather II and III
French Connection II
Aliens (I can enjoy the last two films, too, though good sequels, let alone great movies, they are not)
Back to the Future II and III
I know there are others, but they aren't coming to mind. Certainly there are worthy ones from the Universal and Hammer horror days, and from a few other "franchises" of various stripes.
In general, though, nowadays, the word "sequel" carries about as bad a connotation as "remake".