The best sequels of RoboCop are Total Recall and Starship Troopers.
RoboCop 2 and 3 are equally (very) bad, for different reasons. At least the third one has Poledouris back with a tremendous score. Starship Troopers 2-3-4 are also quite terrible films. Never a good idea to make a sequel/remake of a Verhoeven film.