I'll start: The Stars Wars postlogy was such a disastrous cash grab that it doesn't exist in my head cannon.
This whole thing was a mess.
I think I'm still in kinda disbelief that that happened. Like, maybe don't take me too seriously, but the true sign that the world is going to shit was right there with The Force Awakens. When Star Wars devolved to a totally fluffed up cash grab that we all ate up, that was the sign that we'd lost the plot.
What if the Jedi returned... and then were wiped out off-camera? And the Republic returns, and is blown up by a single big boom boom blast?
What if everything is reset because Abrams bashes other people's toys together while making explosion sounds until they break? 
The Starship Troopers sequels. Which don't exist, but if they did they would be awful enough to pretend they don't.
Halloween.
Executives butchered what was gonna be an anthology series because they saw Michael Myers as a marketable villain. So now every film in the series has gotta be the same thing again and again with the same monster doing the same things, ad infinitum.
Now I come to think of it that's, like, pretty much the entire mainstream of the horror genre isn't it?
Halloween 3 is (imho) worse than Halloween, but it is better than the other sequels because it does try to do something different.
Hot take that might get some shit, but I'll say it anyway:
The Matrix.
The first movie, if it was the only one, was such a sleek tight narrative with such a satisfying ending that left so much open to the imagination that arguably the movie would have more cultural impact today if it stopped there.
The Matrix Reloaded was full of action scenes that still haven't been matched, and I liked the story, even if it was a bit bonkers at times.
The Matrix Revolutions was... disappointing. Big battle scenes had already been overdone at that point, the story was just getting stretched too thin, and dialogue was laughable at critical moments.
Reloaded had some great setpieces, and maybe "worst" didn't really apply to the sequels as the thread requested, but I still feel the original could have stood on its own, maybe timelessly.
I think you're right when I talked about the Matrix and there was worse sequels out there.
I didn't even get to 5 in the Pirates franchise. I didn't even make it to 4.
Oh, jesus, I just remembered!...
The cute but completely and totally unnecessary A Christmas Story Christmas!
Fun to watch once, but definitely not a "rewatch every Christmas" kind of fun movie. So much bad crap just happens over and over again and it just doesn't feel like a happy story!
Most unnecessary, Indiana Jones 4. Last Crusade was a perfect ending. Worst...I submit Grease 2 for your consideration. Not only is it a terrible movie, it is arguably worse than Starlight Express as a musical.
Faraway, So Close!
Wings of Desire is one of my all time top 10 movies, as tempting as it was to make a sequel, it never should have happened. It definitely should not have had a pie fight.
Original:
https://en.m.wikipedia.org/wiki/Wings_of_DesireSequel:
https://en.m.wikipedia.org/wiki/Faraway,_So_Close!
