In an effort to milk every last penny out of a billion dollar industry, the producers of the hit Harry Potter movies are splitting J.K. Rowling’s last installment, Harry Potter and the Deathly Hallows, into two movies. The first film is set for a November 2010 release, and the second for May 2011.

While the Harry Potter films have had nothing but success–worldwide the films have pulled in $4.5 billion at the box office–not all such sequels, trilogies, etc., can say the same. I was pretty disappointed with the later two Santa Clause movies and aghast at the editorial liberties taken in the third installment of the Anne of Green Gables saga.

But I’ve also seen a few excellent series–the Bourne and Spider-Man trilogies come to mind. What are the best and worst movie sequels/trilogies/series that you’ve seen?