Collider's the 10 Best "100th Episode" Episodes on TV, Ranked

"In the old days of network television, a series reaching 100 episodes was a momentous occasion because it meant the show qualified for syndication. It was also impressive that any show could reach such a milestone, especially as the so-called Golden Age of Television began and competition became fiercer.
Shows used to pull out all the stops for their 100th episodes. Some chose to make it coincide with a character's achievement; others used it to bring back familiar faces and take a trip down memory lane. However, the best 100th episodes are the ones that use the occasion to celebrate themselves and their fandom while still advancing the storyline significantly."
-David Caballero