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…
I haven’t seen all of them yet (half way through season 2) but right now I’d say
William Campbell
In … The Squire of Gothos … and also appeared as Klingon Captain Koloth
His acting and persona is equal parts lovable and hateable … the kind of guy you could be friends with while also wanting to stay away from as much as possible.
The other guest actor that I felt the same about was
Roger C. Carmel
In … Mudd’s Women … and later in … I, Mudd
William Campbell is my favorite, too. His characters were all so over the top.
TNG spoilers:
Spoiler
Hey, was he one of the Q? Just never revealing himself?! Playing with this great starship captain for amusement.