Agree with you on the worst prequel being Attack of the Clones. As for best prequel, I actually think The Phantom Menace is because as you said, it's a throwaway introduction piece - there's not a lot riding on it and it does have the single most impressively choreographed lightsaber battle in ALL of the prequels.
Revenge of the Sith is maybe more entertaining than Ep1 but where it fails is the handling of the story, atrocious acting, and unconvincing turn to the darkside. The final lightsaber battle is absolutely boring and unemotional until they stop. The fight was too precise and not enough "close-calls" or "whoa, you almost got me". There was none of that. Just calculated, forced, robot fighting. Nothing human about it.