The Remarkable Steve Ihnat
IMO, Steve Ihnat and Ricardo Montalban are the only two actors from TOS who managed to steal every scene they were in (and in Ihnat's case, steal the scene when Kirk plays Lord Garth!) and that's no mean feat! Consider the scene when Spock is seated next to Kirk and Nimoy is trying (without success) to underplay/undercut Ihnat's over-the-top bellowing with a long pause and condescending "whatever" tone. Nimoy fails miserably and that scene alone is ample evidence that Ihnat was unstoppable in that episode. It also demonstrates everything that was wrong with Spock's characterization is S3, but that's grist for another thread...
"Come, come, Mr. Scott. Young minds, fresh ideas. Be tolerant!"