The second prequel movie is bad. The second original movie is good. The third prequel movie is mostly good. The third original movie is decent.
But what the Star Wars fandom doesn't like to admit is how incredibly close these movies are to each other in terms of being good and bad.
If Attack of the Clones has better actors than Hayden and Natalie Portman (I don't know if she got better as an actress or just went with better roles) to compliment Sam Jackson, Christopher Lee, and Ewan McGregor, suddenly this movie is about on the same level as the others. If Revenge of the Sith removes "From my point of view...", "Only a Sith deals in absolutes", and "NOOOOO!" suddenly that's the best movie in the whole series.
As for the originals, they're saved from mediocrity by being the first technically proficient space opera. Mark Hamill's a good actor NOW, but back then he was only barely more human than Hayden Christensen, and Carrie Fisher wasn't much better either. There's a reason Harrison Ford and James Earl Jones became the two biggest stars out of the trilogy (although the Emperor is pretty dope as well and had a good journeyman career). Movie specific, Empire Strikes Back is weighed down by irrelevant subplots (Luke and the Wampa, 3P0 in Cloud city, Asteroid monster) and Return of the Jedi has Ewoks, the same wooden dialogue from everyone not named Vader, Solo, or Lando, and the Vader turn for no real reason unless you buy into Anakin's "Balance to the Force" theory.
Star Wars has bad movies and good movies throughout, but it's a thin red line that separates them