Empire Strikes Back.
back to the future II. i did like them all though
Aliens over Alien
Manon of the Springs as a sequel to Jean De Florette, which was a good movie itself. They are French movies with English subtitles and a classic tale of human nature and it's consequences.
Terminator 2. I liked it a little bit more than the first one, but they were both great. Sequels are normally way worse.