Well back to the Future III was pretty bad but not as bad as part II.

Rambo III sucked.

Rocky III was okay but not great.

Ghostbusters II was an abomination.

Pirates of the Caribbean III was a piece of garbage.

Return of the Jedi i like so i would not include it in the list but it does have some disappointing moments, it is very uneven kind of like Last Crusade.

To me the last really exceptional Lucasfilms were Raiders and Empire, though i will admit i like star wars '77 better as an action adventure feel good yarn, and empire as a character piece, that is quite depressing actually unless you are an imperial fan.

Superman II is crap unless its following the serious style of part one, as the unfinished Donner version did.

Superman III, was bad because it was a joke film in the style of Lester's other films.

Another reason it was more a Richard Pryor film than a superman film.

And i do like Pryor and he had some funny moments in the film, but i found it inappropriate to the universe as created in part 1.

Part IV was just an embarrassment in terms of the plot the special effects and acting.