A lot of you seem to be from the same, or closer to the same generation. I'm 46 so my list is going to be a bit different than what I've seen thus far. :
Spider-Man & His Amazing Friends
The Incredible Hulk
Thundarr the Barbarian
Dungeons and Dragons
Being a 2000’s kid, I loved (and from time to time, still love)
watching shows like Spongebob, Adventure Time, Fairly Odd Parents, and other stuff airing at the time,but many times I would flip to Cartoon Network’s Boomerang channel and watch Dexter’s Laboratoy, Teen Titans, among many other older shows.
Mine's easy: Alfred J. Kwak. Most of you probably never heard of this. Not so much because it's old, but it was based on a children's musical by Herman Van Veen. Meaning: this was actually something recognizable. This is probably why I still think it stands among the best soundtracks. Not so much because it was better, but because it didn't had to be redubbed or subtitled for children(1). The intro theme for duck tales, for example, is...actually kind of lame when you hear it in Dutch. It's just that as a child, you can't really tell the difference.
Anyhow...Alfred J Kwak was really the one cartoon I really HAD to see (and felt really bad when I missed it). Other things like thundercats and duck tales got me hooked as well, but not to the same degree. Lost cities of gold and Once upon a time...life are also ones I remember liking really much, but not actually seeing.
(1): for those curious: in Belgium we only dub programs/movies for children that can't read yet. I remember at one point that one of the few television channels switched from a Dutch dubbed duck tales to an English subtitled one. While I know it was at an age I could barely read, I had no problems following along.
Every single DC animated series started in the 90's and running into the early 2000's. I am talking Superman, Batman, Justice League (Unlimited,) Teen Titans, Static Shock, and just so many of them. They were all just amazing and most likely why the same studio is being used to this very day to make the DC animated movies.