The Matrix.
I thought the third was a bit disappointing, but I really liked the second one. I'm sure I'm in the minority on that.
Interesting, I thought the third one was okay but hated the second one. Regardless, they both sucked when compared to the original. The Animatrix was cool, though.
Scream 2 sucked, expept when they were talking about how sequels suck. It was like they were trying to figure out "who will the audience never guess it is?"
To further the Aliens thing, I present Aliens vs. Predator (and the upcoming AvP2. I thought Aliens 3 and 4 had some interesting parts but some even bigger flaws.
Highlander 2 is one of the worst movies I've ever seen because it seemed to hate the first film so much.
I really liked Pitch Black and had hopes for Chronicles of Riddick. Bleh. What a poorly written/constructed movie.
The film sequels to Silence of the Lambs (never read the books). Talk about taking a terrifying character and turning him into a cannibalstic Batman.
Batman Forever and Batman Returns. 'nuff said.
That said, I thought the best part of Jay and Silent Bob Strikes Back was when Matt Damon and Ben Affleck made the sequel to Good Will Hunting: Open Hunting Season. That was freaking hilarious.