The highest quality children's show on the television is, without question, Gravity Falls. In a single season it effectively built up an entire world of its own layered with memorable characters, clever jokes, an overarching narrative, countless mysteries, and a myriad of other traits that puts it in a league of its own. And, unlike something like Phineas and Ferb, it doesn't settle for simply pointing out the repetitive nature of children's television before simply engaging in those tropes, but subverts them and gets surprisingly mature at times without ever leaning too heavily in that direction.
GF is the top of the top. Realistically I can understand a case being made for Adventuretime, because I get what it's doing and how it does it, I just don't much care for it, but the clear plans from Gravity Falls that are apparent at the beginning and only further open up on repeat viewings is startling.