Ranking TV shows is tough for me b/c there's a bit of recency bias. When I was a kid, these shows would have been very different. Ask my in another decade, they might be different again. Plus, there are some shows my wife and I watch on repeat, and they're like "comfort food." Others I've seen only once through, and to go back would require some commitment, but they made an impact on me.
Somewhat influenced by
@NIGHTBRINGER, I gave the top spot to a certain animated show...
Avatar: The Last Airbender: What a show! A compelling story with some of the best character development in any cartoon, or maybe any show. The animation is excellent and the story has exciting action, great humor and real heart. The story arc for Zuko alone is worth it. It's a real shame that Mako passed away during the production b/c Uncle Iroh is an anchor for this show's heart. I agree that if you haven't seen it, I think you should give it view.
Parks & Rec: Some great characters and good writing. Funny and sweet. Last season (or last couple?) are not as strong. Give me ALL the bacon & eggs you have. Do you understand?
Batman the Animated Series: THE quintessential Batman for me (sorry Adam West). I know more "realistic" Batman movies are all the rage, but this captured a just gritty enough Batman with the classic comic book flair. Kevin Conroy & Mark Hamill are Batman & the Joker to me. I think virtually every episode is great, but "Heart of Ice" blew me away (it's kind of the "go to" episode for many).
Firefly: What a terrific one season show. The classic ragtag group of adventurers in a really interesting world. It sparked such a huge fan response that a pretty good movie was made. The fact that this show only had one season is wreckin' my calm.
King of the Hill: I'm just drawn to animated shows, I guess. Like most shows, this one really hits its stride then sort of gets old by the end. Regardless, I still enjoy the stories and King's cast of wacky Texans. Bobby is the best.
Breaking Bad: Such a well-written and directed drama. Bryan Cranston is riveting as Walter White and Aaron Paul is great as Jesse (they are interesting foils for each other). But the whole cast is terrific. The drama and tension are
real in several episodes.
Mad Men: Another excellent drama. I'm not sure how much I like Jon Hamm in later movies, but his performance as Don Draper is captivating. So much nuance and nostalgia in this show. The rest of the cast is just as complex. The world of sex, booze, success, failure, and lies all to sell you something really sells this show.
Samurai Jack: Such a unique animated show. Jack & the evil Aku (excellently voiced by Mako) are the main hero and villain. Beautiful backgrounds, a bizarre cast of characters (both villain and ally), exciting action and slick animation are all hallmarks of the show (I enjoy the angular art style). But the sound design and often the lack of, or even the need for, dialogue in several episodes really adds to the allure of the show. Those episodes make you watch (no phone surfing) and reflect Jack's introspective bushido nature.
Cowboy Bebop (animated): Man, I love this anime. I saw it on Cartoon Network's Adult Swim years ago, but it's a triumph of just one season. Spike, Jet, Faye, Ed, and Ein (the corgi) are all interesting. Each alone in there own way, but alone together on the Bebop. Kind of like Firefly, I'm sad it is only one season and a movie, but maybe that works in it's favor. Bang!
X-Files/Seinfeld/Star Trek TNG: Once again, I try to cheat. These shows aren't necessarily interchangeable, but they all are long-running episodic shows that are terrific and made an impact on me. I was late to the game with Seinfeld, but I've caught many on reruns. SO many episodes are just classic and classically quotable. All of the characters are hilarious and just terrible to each other in a comedic way. George is the best... he did win a contest. Even now, the X-Files music makes the hairs on my neck stand up. It played into our love of conspiracy theories, aliens, myth, and cryptozoology. Star Trek: TNG is hands down my favorite Star Trek and the ruler by which I judge other Trek (sorry). Great characters and stories. I thought it really carried on the spirit of the original Star Trek in an updated era (for the time).
TV show honorable mentions (or at least shows that I love to remember): Sesame Street, Mr. Rogers, Electric Company, Reading Rainbow, He-Man & the Master of the Universe, GI Joe, Transformers, Voltron, Thundercats, Teenage Mutant Ninja Turtles, Fraggle Rock, Super Friends, Knight Rider, Family Ties, Growing Pains, Who's the Boss, Quantum Leap, The Wonder Years, Friends, The Simpsons, Dexter's Lab, Gargoyles (What a great concept and storyline for a Disney kids show), Farscape, Brisco County Jr., Lost (Man, those early seasons were great. Curse you final season!), Big Bang Theory, Castle, The Middle, Heroes, Archer, Dexter, Good Eats, Game of Thrones (Curse you final season! Plus, I thought the books were better), Great British Baking Show, Brooklyn 99, Ted Lasso
Whew!... there are just too many to list, but apparently I tried to list them all anyway. Ha ha ha.