My fave is ESB.
I actually would have said RotJ if it wasn't for Ewoks and the DSII! If it wasn't for the most excellent and intense final confrontation 'tween Luke/Vader/Sidious
the movie would have been a bust.
After all, did we really need another Death Star? Sorry, already did the Death Star thing, i.e. the first movie, the big event, the main thang. Need to make up something else cool. It just seemed like a cop-out to me.
And the Ewoks
were just gawd-awful, like GL was sitting around saying, "Well, let's see, what sells? I know! Cute-n-cuddly! Let's populate the biggest movie event of the 80's with blaster-wielding teddy bears!"
Oh, and just to top it off, I thought Sebastian Shaw looked like a dang marshmallow, both in Vader suit and as a ghost. Very disappointing and anti-climactic to me. "This is the guy everyone's been afraid of for three movies?"
Well, definitely burned some bridges here. Laters.