As a follow up to Best and Most Underappreciated Movies in Their Category, I present the following. As with the movie list, you can read "most underappreciated" to mean anywhere from "could be considered the best" to "widely but inappropriately ignored" to "broke the mold and started something new". As an example of that last description, consider the two underappreciated game shows below, Double Dare and The Dating Game. These both were pathbreaking precursors to many future iterations in this genre.
Same disclaimer: This list is not at all definitive; nor do I stand prepared to die on any of these hills. It is my first approximations made and refined with just a bit of supportive research. I will absolutely reserve the right to amend or disclaim any of this as I have only mild confidence that it stands up to more in-depth scrutiny. That said, I'm right on all this, and you know it.
Category - Best | Most Underappreciated
Sitcom (all time) - Seinfeld | Everybody Loves Raymond
Sitcom (pre Seinfeld) - M.A.S.H. | ALF
Sitcom (post Seinfeld) - The Office | New Girl
Sitcom (cartoon) - The Simpsons | (tie) Futurama and Beavis and Butt-Head
Sketch Comedy - Saturday Night Live | In Living Color
Saturday Morning Cartoon - The Bugs Bunny/Road Runner Show | The Smurfs
After-School Cartoon (comedy)* - (tie) DuckTales and Phineas and Ferb | The Rocky and Bullwinkle Show
After-School Cartoon (drama)* - (tie) G.I. Joe and The Transformers | He-Man and the Masters of the Universe
After-School Show (non cartoon) - You Can't Do That On Television | Fraggle Rock
Cartoon (foreign) - Speed Racer | Danger Mouse
Drama - Mad Men | Magnum, P.I.
Drama (gangster) - The Sopranos | Boardwalk Empire
Drama (police/crime) - Breaking Bad | The Wire
Drama (law) - Better Call Saul | Law and Order
Drama (medical) - ER | Doogie Howser, M.D.
Soap Opera - Dallas | Beverly Hills, 90210
Game Show - Jeopardy! | (tie) Double Dare and The Dating Game
*While in many cases these shows were originally Saturday morning shows, I base this on when I primarily watched them.