Happy Days/YMMV

Crowning Moment of Funny: Fonzie's been using "Sit on It!" for years, to great reaction. When Marion of all people uses it on him, the reaction from the crowd and Fonzie himself was priceless.
Crowning Moment of Heartwarming: Joanie and Chachi announce their engagement. Howard, the father of the bride, walks up menacingly to Chachi- "Chachi, I'm going to say something, and I'm only going to say it once: I like you. Welcome to the family." and holds out his hand. Also, when they tie the knot in the Grand Finale, when Howard thanks the audience for being with his family through all these years, and says "to Happy Days."
Ear Worm: C'mon, sing it with me: "Sunday, Monday, Happy Days...".
- The Pumps Your Blood song.
Hey, It's That Voice!: Apparently Ralph Malph ends up stuck in The Realm working for Dungeon Master. Who knew?
The Scrappy: Chachi, but ~~only after he got his own spinoff~~ Chachi was always ~~The Scrappy~~ The Chachi. He was popular with the girls, but retroactively, amongst older fans? Please.
Seasonal Rot: It started out as a very cool, low-key show, then became the ideal version of a standard sitcom with a Studio Audience. It'd started getting worse (the Trope-Named Shark-Jump) even before Richie left, but got apocolyptically bad after that. Fonzie got a temporary full-time girlfriend for one season, and things got increasingly worse with gimmicks and nonsense until a thankfully fantastic final episode. Eleven total seasons, about four or five good.