This is a good idea for a thread! I’m gonna try to pick more underrated choices from each trilogy.
For the Prequels, I would definitely have to say Ian McDiarmid as Palpatine/Sidious. He basically plays two different characters, and he can easily go from kind/friendly, manipulative, to evil incarnate. He’s fun to watch, and his best scene probably has to be the Opera Scene in ROTS.
For the Original Trilogy, I have to agree with Nev and suggest Frank Oz as Yoda. I think the act of performing Yoda was a real pain for Oz, having to not really see who he was performing with, and being cramped underneath the hot stage with his hand up a puppet. But he made Yoda feel real, and made you feel the emotions that Yoda felt, whether it be curiosity, hesitation, disappointment, wisdom, etc.
For the Sequels, it is hard to choose because I think it has some of the best performances in the franchise, but for the amount of pressure I can only imagine he had to have been under, I think Mark Hamill gave his best performance as Luke Skywalker ever, even though he had only acted on-camera occasionally for the past several years. And considering how Hamill apparently didn’t agree with the choices made for his character in the film, he proved he could give the performance regardless of his own opinions, and he really got that pain and emotion across on screen.