Depends on what you consider a cartoon.

I can't pick a single decade either. I'm a fan of the 30's-50's cartoons, in spite of any content/stereotypes that hasn't aged well.

Fast forward a little bit I think the 90's have some of the best cartoons produced, with cartoon network and nickelodeon taking the lion's share of it.

If you consider anime a cartoon too, I would say the 80's is where it's at.