The Marvel-verse is filled with them. Ragnarok, Infinity War, Winter Soldier, Logan, etc.
I think in general it just applies well to any superhero'ish movie since the first movies are usually dragged down by the typical and, unfortunately, boring 'becoming the hero' story arcs.
I don't think it applies to many other genres, where it is rarer to see sequels beat the originals.
X2: X-Men United (2003)
Captain America: The Winter Soldier (2014)
Toy Story 2 (1999)
The Dark Knight (2008)
Star Wars: Episode V – The Empire Strikes Back
I would be hard-pressed to say Terminator 2 is better than Terminator. The original one is also a masterpiece.
The Alien squel in more onthe action side. A good movie, but not exactly the same vibe as the first.
Terminator 2 is the school case of a sequel better than the first.
Batman the Dark Knight for sure is better than Begins (which is already a great movie)
All the ones I originally came in here to say have already been said so I had to really reach deep in the bag for this, but I think there's a pretty good argument for Blade 2
Blade 1 was good in its own right as a straightforward vampire-action movie, but I think Blade 2 was all-around better in terms of story, cool characters, and even the design of vampires having the alien-like jaws was at least new and interesting
But its also been a long time since I watched either of them