Terminator 2 - Terminator one was a great movie and Terminator 2 may arguably be the best sequel ever made. Flipping the script and having the iconic Arnold Terminator be the hero was just an epic idea (Christopher Nolan must have had something to do with it). Terminator 3 and Salvation were just repetitive though the latter did have potential being the first one in the post apocalyptic future.
The Matrix - Revolution was just dreadful. Whether or not Reloaded should have been made is personal preference.
Ocean's 11 -

Any others spring to mind?