Jonny Quest (it took quite some doing to find it on NF, because of the spelling) was as near to a horror show for kids as it could be, given that it ran in the 1960s. Even now, some episodes are creepy! Still, I would not rate it as best, just memorable.
Star Trek (animated) explored aspects of the characters that had been overlooked in the live-action series, such as Spock's childhood.
Popeye will not soon be forgotten; likewise, Betty Boop.
There have been 4 animated versions of Mighty Atom (Astroboy), & the 2009 animated film (though I thought it was not very good) shows that Atom is hardly forgotten, even in the USA, where the sheer number of science fiction / fantasy cartoons and characters makes a robot boy anything but unusual.
I have a fond memory of Hunter x Hunter, even though certain characters' development was a bit sketchy. Likewise, Bleach, in which there are many such characters whose backgrounds just remain unknown. I am currently watching Naruto Shippuden, something like the 276th episode or thereabouts. Here, the series does seem to cover the background of many of the more important supporting characters.
I watched Dragonball, perhaps even to the end, but I doubt it. I think Goku is funny as hell, given his initial state of being unaware of his being a human, & having never seen a female before. Poor Bulma!