The reason I say Peter Cushing, is because of the original question. As a SW mega-fan, the performances of Hamill, Ford, et al are all perfect, and ignited a whole new cinematic world.
BUT, if I try my hardest to step back and view the movies totally objectively, from the viewpoint of a director, playwright, or film aficionado, there is a pretty fair amount of what would typically be called “bad acting” in the OT. I even remember myself (and friends of mine) thinking this in 1977 (but that didn’t stop us from loving the movies). A LOT of critical reviews at the time also gave the film low marks for acting quality, which should come as no surprise given Lucas’ decision to cast mostly unknowns in the lead roles.
However, Cushing’s performance is an exception. At no time is he anything but completely believable. He never looks like he is reading his lines, faking an emotion, or over-acting. His inflection is outstanding and his experience as a dramatic actor are evident. In a setting that is always teetering on the edge of slipping into caricature, he makes the dark world he inhabits real and scary. And whereas some of the actors at times look like they are having trouble buying into what must have seemed at the time like a bad B-movie low mark on their career, Cushing looks 100% immersed, like he took the role very seriously and gave it his Shakespearean best.
BTW, with regards to my “bad acting” comment, above, the prequels make everyone in the OT look like Lawrence Olivier and Ingrid Bergman by comparison.