Alright, let me give you my take on this.....

STAR TREK II: THE WRATH OF KHAN is probably the BEST example of one of the BEST sequels ever made, hands down, and not just primarily because it simply wiped the ass of THE MOTION PICTURE which was a massive bore. Its just one of the finest science fiction films ever made what with its battles between the Enterprise and Reliant, and the whole tension and revenge plot between Kirk and Khan from the original series.

Now, that said, heres my votes for WORST SEQUELS because these always stand out in my mind:

EXORCIST II: THE HERETIC
2FAST2FURIOUS
GREASE 2