Rambo II+III, Rocky II-V
While duds like MIBII or MatrixReloaded were the worst sequels IMHO, the abovementioned were the most disapointing to me, because
Both movies, First Blood (RamboI) and Rocky, have a good script and an interesting plot, evolving characters and even some good acting
The sequels however dump all that quality to concentrate solely on "Sly got muscles, Sly beat baddies up".