Best performance(s) in the OT? The actors portraying the Imperial Officers.
As a poster already noted, they’re very watchable.
Mark Hamill was earnest in his performances throughout the OT, but imo, never convinced as a Jedi Knight in RETURN OF THE JEDI. He was more believable as a farm boy.
Ian McDiarmid’s Emperor was variable. Very effective in some scenes, but corny in others. Never did enjoy the funny voice or cackling.
Carrie Fisher was effective - even when wasted (drugs).
Anthony Daniels and Peter Mayhew probably deserve the most praise.