I think it varies. In the post-World War II decades, America had the advantage until the 1970's. Japan started to get better and by the 1980's, Anime blew away anything that Americans were doing. Then Americans made a comeback made a comeback in the 1990's when Disney had their renaissance and you had cartoons like Batman The Animated Series and X-Men The Animated series. Television cartoons had the high production values that were normally reserved for animated feature films. This lasted until the early 2000's. I think one of the last great cartoons from this era was the 2003 Teenage Mutant Ninja Turtles series. After that, cartoons got more simplistic and later on, many animators started abandoning hand-drawn animation in favor of CG, which for the most part looks crappy on television. So I think Japan is ahead again because their animation has only gotten better over the decades and has not yet jumped the shark.
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