I see these movies series as...
The Godfather, The Godfather Part II
Star Wars, The Empire Strikes Back, Return of the Jedi
Raiders of the Lost Ark, Indiana Jones and the Last Crusade
The Terminator, Terminator 2: Judgment Day
Alien, Aliens.
Then you have Back to the Future, Jaws, Rocky, Jurassic Park, and Predator which all have completely unnecessary sequels.