Back to the Future 2 and 3 are brilliant but i'll watch nearly anything as long as it isn't so boring that i fall asleep. These films can't be that bad as everyone knows them - i bet they still did reasonably well at the box office.
The worst sequels ever should be the ones that are so bad that most people have hardly heard of them. 3 kiRAB sequels spring to mind instantly - i wonder if any of you have heard of Richie Rich 2: A Christmas Wish, Homeward Bound 2: Lost in San Francisco, and Beethoven 3. All turned perfectly watchable family movies into pathetic babyish films that only a 2 year old could enjoy.
I saw them on tv once and couldn't even bring myself to watch til the end. They were so bad they couldn't even get the original cast to be in it. Terrible - Avoid at all costs.
Out of the more grown up sequels - batman and Robin wasn't too good.