1. The butterfly effect II (2006)
A second, and even a third movie, which they actually made, was completely unnecessary. Rather than being sequels, the two subsequent films turned out to be a repeat of the same movie with different actors, and it got worse with each retelling.
2. The hangover part II (2011)
As I am not a huge fan of comedies, I found the first Hangover movie a really good one and I could say one of my favorite comedies. But... the part II is just a bad copy of the first movie. Replaying the same story but in this case in a different city, same jokes etc. But this time it wasn't even funny...
3. Pocahontas II (1998)
Truth is that I haven't even watched the whole movie. If you are a fan of the Disney 90's movies like I am then probably you will be completely disappointed with it. After a whole first movie about the war for the love of Pocahontas and John, in the second movie she falls in love with an other guy...Really Disney? And no, I don't care if the second movie was more historically accurate.
4. American Wedding (2003)
Same as the Hangover, the first American Pie movie is a good comedy. A big negative for the American Wedding is that one of the main characters didn't even participate. Some of the joke scenes of the movie are even disturbing.