For me it's a bit of a toss between 4 and 5.
I think we can all admit though, all 6 of the movies are flawed to some degree. Yes, even the beloved 4, 5, and 6. You can take a farm kid, drop him into a military grade ship, and he flies circles around the "vets"? Armored ground troops are beaten by teddy bears with sticks? Our ships are being fried by a giant moon sized laser? Hmm, I wonder if we should fly around to the backside of that moon where the laser isn't. Artoo can fly? Oh wait, now he can't.