The thread below about top flight actors who made appearances on Columbo got me to thinking... I would have loved to have seen Christopher Walken in an episode. I think he'd have made an excellent Eric Mason ("How to Dial a Murder"), even though that may have been a bit before he reached megastar status.
On the previous thread, although he was 20 years past his most famous roles, I was always struck by Vincent Price's appearance in "Lovely But Lethal" - he was a true cinematic giant, regardless of the time he appeared. I would have enjoyed him as a murderer in a Columbo episode.