Okay, so here's my ranking:
V III IV II VI I
It's based on the idea of "does this movie hold my interest all the way through?" III's one of the best for that. Would it stand up to repeated viewings? I have no idea, but if I can sit through multiple viewings of I (ie: for the commentary), I can handle III again. Heck, I'll probably notice a bunch more things.
The funniest part about all this is that George Lucas really thought of the prequels as something to exist in the imagination as opposed to being something real. This all came from a little joke he made in homage to the Flash Gordon weekly serials of yore; in 1974, you were supposed to come in and feel that you may have missed something (As Lucas says on the commentary for IV, "Sorry, you missed last week's show."). Making these prequels must have been a daunting task. After seeing III, I feel amazed that they pulled it off so well. Really, I exists to set up II, which sets up III, which in turn sets up IV... so maybe that's part of the reason why I had to be a little slow to get started.