I’m going with William Windom:
https://www.youtube.com/watch?v=lS7cK-mQ0fQ&t=134s
Staving off reality:
https://www.youtube.com/watch?v=qmnpz6TheSs
Doing a villain turn by seizing control:
https://www.youtube.com/watch?v=83tW0jhEvZE
Matt’s heroic sacrifice:
https://www.youtube.com/watch?v=U_mwtoeqneM
Finally, one of the most nail-biting scenes ever:
https://www.youtube.com/watch?v=DHmsnQ8WVpw
Personal notes: across all TV and media, I’ve rarely seen a character go from totally sympathetic, to total heel-turn villain, to total martyr like that.
In terms of ST series, maybe Dukat from DS…?
Runner-ups?
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Mark Lenard as the Romulan commander
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Robert Walker as “Charlie X”
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Roger C. Carmel as “Harry Mudd”
Daggit, I know I’m forgetting some actors, here…
Not going to state the obvious with Ricardo Montalban, but I always enjoyed William Campbell’s Trelaine. And yeah, Windom was solid as Decker. Great episode all around.