I'd say the award for worst acting in the original Star Wars film goes to the guy who played "Mauler" Mithel, a.k.a. Black 2. I guess I should blame the director, but his facial expressions were completely unreadable. I just couldn't make an emotional connection with him.
I thought he sold his death scene pretty well.
"Look out!" <BOOM>
Threepio, on the other hand, shouldn't be allowed anywhere near a set. He has the facial range of a hunk of metal.