Well "The Thundercats" is my favorite from my childhood.
For modern ones I'd probably have to also give a nod to "Avatar" even though I've only recently started paying any attention to it.
I am not including Anime in this catagory since people tend to interpet Cartoons as western and anime as eastern.
There are some interesting adult and prime time cartoons out there, but truthfully they tend to get old, and pretty much recycle the same bits after a while. A lot of the Adult Swim lineup seems to do the same basic things with differant characters on differant shows after a while. It's still entertaining mind you, but it prevents me from calling any of them 'the best'.