I know this is going to create a stir with the true Star Wars fans, just like saying the LotR films are better than the books, but personally I prefer the three later films. Okay I'm a sucker for eye candy, and being made twenty years after the originals the new films have a great advantage in that department. But it's not just the eye candy.... I think the later films are acted better too.
I'm not a big fan of Phantom Menace, mainly because of Jar Jar Binks. If there is one thing that bugs me about Lucas, it's that stupid slap-stick element he insists on putting in his films. Doesn't anyone else get tired of seeing 3CP0's head stuck on back to front, or attached to a bad guy? It's a joke that probably raised a laugh in one film, but did not need to be relentlessly carried on as a theme in the others.
That is what I liked most about Revenge of the Sith.... he had made the film darker, and left out that slap-stick, cheap laugh humour. I'm not a huge fan of the series, but Revenge of the Sith I really do like. Lots of action, lots of eye candy, lots of turmoil, some great acting.... and it finally answered the question of how he was going to complete the circle to arrive back at the start. To me, that one film stands out so far from the crowd.
I'll stand back now and be bombarded by rotten apples from the true fans.