Richard Brooks, as the deputy assistant district attorney on the early Law & Order. He was on for the first couple of seasons, I think. They haven't had a man in that role since.
June Lockhart. One of the great mother-role character actresses on TV. Maybe its just the nostalgia talking, but she was great on Lost In Space and Lassie.
Martin Starr as Bill Haverchuck, Jason Segal as Nick Andopolis, and Joe Flaherty as Harold Weir (the father) on Freaks and Geeks. The whole cast was great, but those three really impress me, right now at least.