petertrek1, 
A year and a half ago, I was a guest on the TrekRanks podcast discussing "Top 5 One-Off Guest Stars".
My #1 pick was Harris Yulin from "Duet". Highlights of his performance were his conversations with Kira/Nana Visitor. There were riveting to watch.
http://www.thetricordertransmissions.com/episode.php?ep=TR131