You can't possibly believe this.
You know who Lucas is, right?
Episode III WAS the best of the prequels, but look at what you're comparing them to. Anakin's fall was still not very believable, Hayden Christansen and Natalie Portman failed at any kind of romance/drama, parts of the writing were just as terrible as Ep1 and 2, and bits of it, even the ACTION SCENES, dragged on.
If it didn't have Ewan McGregor to *********** PWN that last scene with Anakin (and the scene with Greivous) it would have blown period.
But the entire PT would have blown without Ewan.