While it's nice to see Robert Donat remembered on this list, this blog would vote for Sally Kellerman from Back To School. Her sultry reading of Joyce is worth the price of the rental alone. (Actually, I saw it in the cinema when it came out, which tells you something about how old I am. It's a little hard to believe she was almost fifty at the time.) As Rodney Dangerfield puts it, a little star-struck by Ms Kellerman: "I like teachers. If you do something wrong, they make you do it over again."
BTW, she also does an a rather good reading of Thomas' "Do Not Go Gentle...," which summons Rodney, stirring back from exhaustion in a series of oral exams, to offer an exegesis that would have made Dylan laugh: "It means... I don't take shit from no one!!!" Make sure to follow the link to hear Thomas' reading; it's justly famous.