On any other board I'd say Land Before Time, but I figure you'd like a more interesting answer
The first candidate that comes to mind is
Batman the Animated Series as it's one of my favorite iterations of the Batman character. Writing was great, many characters like Dr Freeze and Clayface had their identities crystallized from this show, and it gave birth to Harley Quinn who's now one of the most famous members of the entire rogue's gallery. However, I hesitate to declare it my favorite cartoon ever as it's just one part of the Batman franchise, which is what I'm truly a fan of. It's a commendable part of what makes me a Batman fan, but there are so many elements and reasons that extend beyond this one show.
So I think I'll have to give my vote to
Scooby-Doo in all its runs. The core Scooby gang is iconic, with each bringing a different thing to the table, and for a saturday morning cartoon the mystery element was reallly refreshing. Shows back then tended to be light, fluffy, and wacky, so the spooky decrepit caves and run-down ghost towns was a really different vibe from what else we had to watch back then. The mystery element furthermore required an element of cleverness on the writer's part, using either misdirection, scientific explanation, or detective work for the protagonists to resolve the plot, and again this was a stepup from the usual cartoon writing that otherwise relied on laser guns, wacky hijinks, or the power of friendship to resolve problems.
Mystery Incorporated is my particular favorite iteration, one of the best examples of a 'series reboot' I've ever seen, with fresh takes on the characters that heighten each of their narrative roles (it invented the now-commonplace interpretation of Fred being enthusiastically addicted with traps, a much-improved characterization over the 'generic everyman leader guy' personality he had before).
Edit: I suppose
My Little Pony: Friendship is Magic deserves a mention too, as it had a huge impact on my life in multiple ways. Anyone online in the early 2010s remembers the insane wave of popularity that swept the internet when that show first released, and my boyfriend and I capitalized on it to get our first game-writing jobs for
Legends of Equestria. It wasn't an accident that show was so popular; the core 6 characters were incredibly well-defined, with unique personalities that even the most casual fans could understand at a glance. The writing was above-average for 2010s cartoons, and without trying it bridged the gap between male and female-oriented cartoons. Even today, its impact is everywhere in the cartoons being released today, you can look at shows like
Steven Universe, [Star Vs the Forces of Evil[/i] and the new
She-Ra and see them capitalizing on the trail MLP:FiM blazed. Furthermore, it has probably my favorite cartoon character of all time, Starlight Glimmer, who I really related with due to her resourcefullness as a fighter, her mastery of emotional/behavioral manipulation as a villain, her checkered redemption arc that really put her through the ringer over the years, and her social awkwardness despite her extreme technical competence. These are four different character archetypes I'm a huge fan of, and MLP:FiM crammed them all into a single character.
