I was thinking about this earlier, and honestly, there's no real difference in quality between Episode III, at least, and Episode VI. Arguably Episode IV, but that one's kind of a different issue so I'll focus on VI.
Both have problems with acting and writing, and III arguably has the lesser issues there. III is better paced for sure, and while the Luke/Vader conclusion was outright excellent, VI sidelined the development of the rest of its cast while III dealt with a more complex narrative and cast and did it mostly pretty well. There were painful lines in both films, and really good ones. Both films did action really well but you have to give III the nod when comparing the two. Portman definitely gave the worst performance of either movie, but Fisher wasn't exactly a star in VI, because drugs will do that to you. Overall, both films were compelling if occasionally very silly sci-fi movies that had good plots and excellent action. Flawed, but with stuff to recommend for both.
So yeah I suppose what I'm saying is I just called a prequel equal to an original and there's nothing you can do about it.
And honestly, I and II weren't horrible either, though I will say that I was pretty much nothing like Star Wars. Definitely the oddball of the series on reflection. But yeah, they were mediocre at worst.
Edit: Battle of Coruscant was great, even if the focus wasn't on the whole battle. But that was kind of the point, it was supposed to be a POV scene. Also II had an asteroid field chase as well. That was okay.
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^Poorly disguised anti-caps sig