Actress - No doubt whatsoever here - Nancy Barrett. Effortless acting from the beginning until the end. She played so many different characters, and they each had their own manner and personality traits. Her performances were always strong, and no matter the emotion, she could play it and you believed it was real. And I've always been fond of Kathryn Leigh Scott, for the same reasons Willie mentioned. She always seemed very natural and I liked that.
Honorable mentions go to Joan Bennett (who, when she had the material to work with, was great), and Lara Parker (who always seemed intense and into the role).
Actor - No doubt here, either - Jonathan Frid and John Karlen. There was so much more to Frid's portrayal than just the lines: his presence, his voice, his expressions. The depth of his portrayal was mesmerizing. Even in the black and white episodes, when he was just supposed to be the villain, there was so much more there and a lot of that was due to Frid's performance. With John Karlen, his turn as Willie alone was amazing. He gave great performances as different characters, but the evolution of Willie proved his range. Both Frid and Karlen bring the emotion out in me - I feel so much for their characters when I'm watching the show. That says a lot about their acting, IMO.
Honorable mention goes to Louis Edmonds, who was always convincing and amusing as Roger, and gave a powerful performance as Joshua.