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?

• Mark Lenard as the Romulan commander

• Robert Walker as “Charlie X”

• Roger C. Carmel as “Harry Mudd”

Daggit, I know I’m forgetting some actors, here…