I enjoyed KOTCS at release and I still enjoy it now. Sure, it's flawed in many places, but it's a film I can go back to and have fun with. The prequels have their moments but overall I still find them mediocre and dull to sit through. ROTS is probably the best but it still suffers from poor writing, bad acting, and over-indulgence, particularly near the end. Phantom Menace has a few great moments with a lot of nothing in-between, while AOTC is embarrassing, though I do like some of the world building that it introduced.
For me it's no contest. KOTCS still gives me reasons to come back, while the prequels only get viewed when my wife and I are having a Star Wars marathon. I've softened on them a bit over the years but I can't see them ever becoming favorites.