I'd have to say Revenge of the Sith ROTS may have its flaws but it featured characters I had some investment in. It was the culmination of an entire saga featuring events we had waited to see since the very first movie alluded to them. It might not have been perfect but there's a sense of pay off. With RO I never grew too attached to the heroes and they all end up dead at the end anyways. It just didn't leave the lasting impression that ROTS did.