Fair Warning, I'm not huge on Star Wars at. Haven't seen 2 or 3, and I don't even feel the need to cover 1 (it's not as good as 4-6). Here's just my personal opinions considering the original, but they can't be real appropriate to "real" fans.
4 was a great opening for the (original) series to gain footing, but there was no real plot to it in itself. All it really did was introduce everyone, nothing to make it the best. Also very actionless for the most part, and standalone story and elements were lacking (made up in the further movies though).
5 is the best that I've seen. It has a large bulk of the original trilogy's plot devices (Jedi training, Luke/Vader connection), and had a great deal of action too. It even had sappy emotional stuff like the Leila (?) and Han relation (followed by his encasing). That's the "must see again" movie of the series.
6 was alright. Much like 4 though it didn't really do much in itself, but rather just finished the unfinished. I found a lot of the elements of the movie dull like Luke's squirming for freaking ever and a lot of other useless stalling (the opening was great though). Still a good movie, just not stand alone.
Since some of my points don't seem valid or justified, I'm basically trying to say that 5 is a real "movie" while 4 and 6 begin and end the storyline appropriately without being 'real' movies.