Vox.com has a list of 25 Episodes that Changed TV. Here are some of the highlights:
1. "Lucy is Enceinte", from "I Love Lucy" (1952): CBS didn't want Lucille Ball's real-life pregnancy written into the show, but she and Desi Arnaz wanted the show to reflect their lives. And it brought a new realism to how TV characters were depicted.
2. "Plato's Stepchildren", from "Star Trek" (1968): William Shatner and Nichelle Nichols gave us TV's first interracial kiss.
3. "Abyssinia, Henry", from "M*A*S*H" (1975): When Lieutenant Colonel Henry Blake got his discharge, they could have just let him go home and lead a happy, post-war life. Instead, they had him die in a helicopter crash on the way. At the time killing off such a major character was a pretty bold move.
4. "A House Divided", from "Dallas" (1980): This was the infamous "Who Shot J.R." cliffhanger. The eventual reveal might have been a disappointment, but the mystery literally gripped a nation.
5. "The Puppy Episode", from "Ellen" (1997): The coming out episode. Enough said.
6. "The Final Four", from Season 1 of "Survivor" (2000): This is the episode where Susan Hawk gave her infamous "Snake vs. Rat" speech, and the jury decided it was better to vote for the backstabbing dirtbag who didn't hide the fact that he was a backstabbing dirtbag.
7. "Baelor", from "Game of Thrones" (2011): "M*A*S*H" may have opened the door for the killing of a major character . . . but this Season 1 episode of "Game of Thrones" killed off the MAIN character . . . Sean Bean's Ned Stark. It was a pretty legendary move, and it, quote, "sent the rest of television scrambling to come up with ways to top it for shock value."
Full List Here https://www.vox.com/a/most-influential-best-tv-episodes