I think the cruelest part about this question is that it's hard to whittle it down to one. So have a mess of them.
Samurai Jack isn't twenty years old yet, but when it will be, it'll take top place. In the meantime, at a very close second, is Batman: The Animated Series. Both of them are very well constructed, from the animation and stylization, to the story craft. They're well thought out, well paced, engaging, and often relatively creative. Both series have many episodes that stand out in my memory as amazing and I'd be hard pressed to think of an episode I didn't enjoy.
MASH and The Andy Griffith Show both stand out to me as being television sitcoms that explore a very wide spectrum of morals and philosophies, and while they don't always do so with depth, there's always something that can be taken away from them. The Dick Van Dyke Show and The George Burns and Gracie Allen Show are both just a whole mess of fun. Everyone very much seems to enjoy what they're creating together and the energy and enthusiasm can be felt in almost every scene.
Have Gun - Will Travel, The Rebel, Trackdown,, Wanted: Dead or Alive, pick one for me, I'll probably agree with you.
Nova. There is literally zero, zero chances that you can pick any episode of Nova, watch the whole thing, and then look at me with a straight face and say you've gotten nothing out of it. This series amazes and inspires, it thrills and fascinates, it deepens and enriches. It's good, good television.
Bonus Random Stuff
Get Smart, The Adam West Batman Series, Animaniacs, Sam and Max, I Love Lucy, Allo Allo, Barney Miller, Antiques Roadshow, Tom & Jerry, Looney Toons, and so many more.