Wow, there's too much of them. The real question will be what movies had sequels as good or better than its prequel.
Anyways, to your question.
Twilight definitely. Well, I don't like any of the movies but I did find the first movie bearable while the others were utter disasters.
Same for Matrix - I don't like the Trilogy but while the first was decent, I hated the 2 others.
X-Men series - I like only the first and the Wolverine one.
Pirates of Carribean - the 4th one was meh~
Harry Potter - Well, I'm a big fan but if I were to choose some that I like less, I'd say Half Blood Prince mostly. Prisoner of Azkaban, Goblet of Fire, Order of phoenix were okay movies while the others are better with Chamber of Secrets remaining my favorite.
American Pie - The spin-off are far far from matching the Originals.
There are too many to sequels unfortunately that I can't analyse all and many were less good and many were disasters too
If I were to state a succesful series, it's for sure the Lord of the Rings where all movies had very good caliber and Nolan's Batman series.