Either A New Hope or Return of the Jedi. Until Episode I came out, I liked Empire Strikes Back the least.
Episode I unfortunately was meant to be a kid's movie. Hook them when they're young to maximize the merchandising potential, then slowly ramp up the violence as the audience gets older.
Episode III was alright, but it was rather depressing, what with all the Jedi being killed. Depressing in a different sense too was their pathetic defense against the Clone Troopers. These were the Jedi who survived Geonosis, so you'd think they'd have some good skills, but apparently they can still be shot in the back quite easily.
Empire Strikes Back was a downer in a similar sense.