Luke's, easy. It may be considered overacting by many, but I think it fit. I wouldn't expect him to get his hand cut off, be literally hanging on the precipice of death, then find out his mortal enemy is his father and everything Ben said (the foundation on which he even began his journey in the first place) may very well be based entirely on lies, and react subtly. It just doesn't happen. He was in extreme pain, then had the very foundation of everything he had come to believe shaken to the core. I think his reaction was fitting. It sounded like he was in extreme physical and emotional pain.
Darth Vader's was more overdone, partially because it was entirely generic (Luke's was broken, raspy, and he even had a smaller "no" after the big one, whereas Vader's was simple, consistent, and outright "performed"), and partially because he literally leaned back and raised his hands to the sky. That's overacting, though it's actually more likely to just be bad directing.
