My vote goes to Leonard Nimoy in A Stitch in Crime, his performance makes me cringe so much I almost can't watch that episode. Especially when the character goes into a fit of giggling when Columbo accuses him of planning to kill Dr. Heidemann and warns him that he'll have to take very good care of him. Both that scene and his acting pretty much throughout the episode (including the Vulcan death grip he uses to fell Harry Alexander) are SO forced.
BTW I'll never understand why this episode isn't slated more than Dagger of the Mind, I actually find the latter very enjoyable!
sorry...I'm somewhat new to site and reviewing some rather old posts....but am loving the detail of those said posts
as for worst actor/actress...the difference between Ray Milland's two appearances is night and day...fantastic in Death Lends a Hand and lame lame lame in Greenhouse.
My fellow Canadian Larry Mann's performance...Albert... in Murder Under Glass was very wooden I thought.
other mentions
Shirley Blaine..Lovely but Lethal
Ruth Stafford (Collin Wilcox)...Exercise in Fatality
Lewis Lacey ..(Darrell Zwerling)...Exercise in Fatality
the boy genius...Mayhem
Janie Brandt ...Jeannie Berlin ...Old Fashioned Murder