It may be due to my age relative to the films, but I've always considered Empire Strikes Back to be the best of ALL the Star Wars movies, original or prequel. Return of the Jedi was very good also. I have always found the
original original Star Wars (later known as Episode IV: A New Hope) to be very slow-paced, cheesy, and boring. Sure it has it's moments, but overall it is a dull film and the dialogue is awful even by Star Wars standards.
I'll revise later if I think of a better candidate, but in my opinion Terminator 2
is in fact the best unplanned sequel ever made. You could make an argument for Lord of the Rings, but since those films were all planned and made together, it's difficult to argue them as "true" sequels rather than one really long movie that is broken up into parts for the sake of practicality.
Not to derail the thread, but one of my biggest beefs with more modern films is the complete reliance on CG. What made older movies like Terminator 2 great is that CG, when used, was used in a way that enhanced the film, and couldn't be done in any other way because there was no basis in reality for what was being represented (obviously speaking of the T1000 here). Anything that could be done through stunts or pyrotechnics, was! In fact if you watch the director's commentary it really gives you a new appreciation for the film. The helicopter flying under the overpass? Real. The semi crashing off of the road and into the canal? Real. The cyberdyne building exploding? REAL!!! (that building was scheduled for demolition anyway

)
You just don't see that quality or effort in films anymore, it's all computer generated, and it shows.