Best male guest star?

Patrick Magee as J.P. Spagge in "The Gilded Cage" (already on a wheelchair eight years before "A Clockwork Orange").

Peter Wyngarde in A Touch of Brimstone and Epic. He had a wonderful sense of humor and delivered each line deliciously. Also Patrick Cargill in The Murder Market and Fear Merchants. Another actor who clearly was enjoying his time on The Avengers.