Personally, I dont think any movie (yes, including shit like From Justin to Kelly and You Got Served) can be worse than certain sequels. These are sequels that bring down great movies.

Oceans 12 was a great example. I cant enjoy Oceans 11 because of it. Its that bad.

I must also bring forth all three Star Wars prequels. They have officially made me lose respect for the entire franchise. I saw all the old ones 60 times when I was a kid, probably wont watch them again. The prequels got me over Star Wars forever.

Others include

T3 (makes T2's amazing ending pointless)

Jurassic Park 2 & 3 (oh wait theres more dinosaurs on ANOTHER island)

The Austin Powers sequels.

dozens more