M*A*S*H
Younger people may not be familiar with this series, but it was one of the most popular in the 70s. If not the most popular. The love of American viewers for this comedy set amid the Korean War was such that the final episode, "Goodbye, Farewell and Amen", was watched by 106 million viewers (77 % share). A number that no other series has managed to achieve ever since.
Dallas
The one big question in this cult show had been for a long time: who shot JR? Which character is the culprit? The answer came in episode 4 of season 4 titled "Who Done It". In November 1980, they were between 83 and 90 million fans watching the outcome of this exciting storyline. The most watched television episode at the time... to be beaten by the final episode of M*A*S*H three years later.
Game of Thrones
While the last season of the epic series disappointed many fans, the last episode broke all ratings records on American channel HBO. 13.6 million watched the 6th episode of Season 8 live, and that number rose to 19.3 million including on-demand replays and legal streaming. No other series on HBO has reached such a number.
Friends
Definitely, series finales are popular. While Friends may not be the most watched in series history, it was the most viewed episode of the 2000s, with 52.2 million viewers. A real achievement for a show that is still one of the most watched today. For the record, the most watched episode of Friends is the double episode 12-13 of season 2, aired after the Super Bowl, with 52.9 million viewers.
Empire
The musical series that landed in 2015 broke a record not on a single episode, but an entire season. At the time of its broadcast, it managed to bring down a 23-year-old feat. How? By setting week after week a better record than the previous one: 9.90 million, then 10.32 million, 11.07 million, 11.36 million, 11.47 million, 11.96 million, 13.02 million, 13.90 million, 14.33 million, 14.90 million, 15.82 million and finally 17.62 million viewers for the final episode of Season 1.
The Walking Dead
Before the arrival of Negan and the fall of ratings, there was no doubt about the popularity of The Walking Dead. Viewers grew in number season after season, reaching its record viewership in the first episode of season 5 with 17.29 million. A historic performance for a show that aired on American cable, at the time, before the last episode of Game of Thrones aired on HBO. And that's without counting on-demand replays and legal streaming.