Men in Black II - the first was hilarious, the sequel was just lazy and completely undid the moving aspect of the end of the first film.

But let's not be just negative, can people also think of sequels (or even third films) that they really like, or which surpass the original even? Leavng aside LOTR, which is essentially one film divided into three in my opinion, my suggestions are:

Back to the Future Parts II and III (also filmd back to back like the Matrix sequels, but with far more successful results)
Toy Story 2 (along with Godfather Part II, one of only two sequels that is better than the original in my opinion)