TOS: Operation-- Annihilate!, streamed on Netflix.
Another great first season episode, and shows a wonderful example of the deep friendship between Kirk, Spock and McCoy. Superb performances by Nimoy and especially Shatner, as Kirk has to deal not only with the death of his brother and sister-in-law, the potential loss of Spock and his nephew, Spock being blinded and McCoy's guilt over blinding him, but also with the prospect of having to implement GO24 for real and destroy the one million colonists on Deneva to halt the spread of the pancakes.
This episode contains what may be the first ever instance of lens-flare in Trek, at around the 7:55 mark.