I like sequels when they are done to continue a story and not meant to revisit the first one over and over again. I think a lot of people see sequels and expect them to be the first all over again, but don't realize people have to change, grow, evolve. When they are shot sequentially, like LOTR, and meant to be one movie, then they are the best. It's like Kill Bill 1 and 2. A lot of people didn't like the second, but he shot the film as one piece and cut it in half after filming. It's really one movie.
The best sequels are Bourne Identity and Supremacy, Indiana Jones, Before Sunrise/Before Sunset. You can't even insult things like Harry Potter because each one is based on an individual book. It's not like the first was made, did well, and so someone said, let's keep going with that until we mash it to death.
The worst sequels lose the integrity of the first. My big example is The Crow. As far as I will ever be concerned, part 2,3, and 4 do not exist. Those are sell outs..