I remember either voting in this, or it might've been a completely different sites list. Anyway, this was published by IGN some years ago, therefore I will create it here and now for list purposes. From the official IGN release:
The cartoon is indeed a wonderful thing. Once mostly deemed to be the territory of a kids-only audience, the world of animation has evolved over the years so as to appeal to adults as well as children… or is it just that many adults these days have the minds of children?
Don't ask us, for if there's one thing that the IGN editors can claim to be, it's kids trapped in the bodies of adults. And so it goes without saying that we sure do love us some cartoons. And in particular, television has proven to be a hugely important part of our toon-development, starting from our earliest days with the Saturday morning and after-school shows and continuing right on to today with the more mature Adult Swim offerings and the like.
In fact, as we prepped for this story and looked back at our many favorite animated series from over the years, we were amazed by the diversity of the shows that we came up with. From classic tales of cats chasing mice to the legend of a Dark Knight avenger patrolling the streets of a place called Gotham, from incompetent alien invaders to incompetent nuclear families, from stories of the future to sagas of the past, IGN's Top 100 Animated Series has it all.
So put away your ink and paint for a while and have a look at this list. You might just find yourself animated by it...