GUEST, that episode with Lee Marvin and Lee Van Cleef is a great one. James Best (The aforementioned Dukes of Hazard sherrif) is the fourth guy in that one. Best also has his own episode, where he plays a guy who dies and comes back.
Burgess Meredith was one of the Zone's favorites, appearing in four episodes. I believe Jack Klugman also did four.
Theodore Bikel is a great one because of his dual roles as actor and folksinger. He also turns up in Star Trek (as the Jewish Earthman who raised Worf) and in Babylon 5 (as the Russian Rabbi who is a friend and religious counselor to Susan Ivanova; and also as a Minbari religious leader.)
That reminds me: there is a rarely-shown episode of the Twilight Zone featuring Robert Duvall. It's rarely shown because it's one of about fifteen episodes that were an hour long, and the show was syndicated as a half-hour show in reruns, so those episodes are frequently skipped. If I recall, Duvall is obsessed with a dollhouse in a museum.
It just occurred to me that the old Batman is a great place to look for this phenomenon, too. Not only is there the show's penchant for big stars of the time as villains (David Wayne, Burgess Meredith, Vincent Price, Eartha Kitt, John Astin, Victor Buono, etc), but there were the people who would stick their heads out the window while the dynamic duo were climbing buildings (from Milton Berle to the Monkees).