It seems to be a general trend in film that when a successful film gets a sequel, nearly always it ends up being worse than the original. However every so often an exception is made where the sequel actually improves on the original. For this I'm not counting films that were intended to be a series from the start (e.g. Star Wars, Back to the Future, series based off books, The Godfather etc) but sequels that were made due to the success of the original.
Here are my picks:
Aliens > Alien
Shrek 2 > Shrek
Die Hard 2 > Die Hard
Kung Fu Panda 2 > Kung Fu Panda
Iron Man 2 > Iron Man
Toy Story 3 > Toy Story 2
That's all I can think of now but I'm sure there are loads more!