"TOS: Let that be your last battlefield" is one of the most powerful TV episodes I ever watched. A similar story can be seen in the Night Gallery feature "Class of 1999".
(you can probably tell I love morality plays)
Never watched any of the sequel series beyond TNG, although I did watch the Tribble episode of DS9.
I would agree some of the later TNG season finales were pretty badass. Loved the one where Data joined his bother. And yeah, when Riker unflinchingly looked his old captain in the eye and coldly said "Mr Worf...FIRE" that was a massive 'holy s--t' moment.
Admittedly, those were made more fun by the fact that at the time they originally aired, I was dating a hardcore Trekker and she would totally geek out at those moments