I think Star Wars is the best by a hair, Empire is better in a lot of ways just not as fun. Return is the weakest and uneven but still has its moments.
To me all three films, i mean the real versions are one piece. They tell a complete story i find it hard to consider them separately. Although i know Leia was never intended to be the sister, or Vader Luke’s father. The story of the original film when viewed as a standalone thing is very different from what came later. Star Wars itself not the SAGA is a different beast. A simple happy go lucky comic book movie, not some grand Epic.
A movie serial like Buck Rogers or Flash Gordon. Tells a simple story and is self contained.