I prefer the originals in all these cases (with the exception of Toy Story 2), but I do agree that they are all terrific sequels. You also missed the most obvious one: The Godfather Part II (which I consider better than the first). I also agree The Empire Strikes Back is a better film than Star Wars. See also X-Men 2, Mad Max 2 (known as The Road Warrior in the US) and Star Trek II: The Wrath of Khan.