Plldwn: this is a great thread - thanks for starting it.
Reading all the responses has reaffirmed my belief that the acting in the OT was very high overall for films of this genre. My gut reaction to the initial question was Mark Hamill for reasons already stated: mainly his wide range of emotions and progression in character maturity throughout the trilogy. But there are others I would place alongside to whom I had not given much thought until now. I agree that Anthony Daniels as C3PO was a real standout, and not only for his perfect delivery. Despite the limitations of being hidden away inside the robotic suit, he managed to convey great expression in the way he walked and moved. His body language during the various exchanges with R2D2 in ANH and Solo in TESB brings the character to life. The imperial officers were also all well cast. I thought Michael Sheard (Admiral Ozzel) and Jonathan Glover (General Veers) were especially memorable, although their roles were arguably too small to qualify them for “best overall actor”.