While I disagree with Alex's high assessment of The Lion King (Which I think was only a so-so example of animation, some of its best scenes borrow heavily from other movies), Disney's early output stands at the top of early 20th century animation. Disney cared too, he cared about pushing the boundaries of his medium to the limits, I would personally consider Fantasia his best work though, and better than everything the studio has produced since his death, even if it has less stellar moments. Current Disney Studios are much less impressive.
There are better animators though, certainly those with much less popular appeal. Disney was a trailblazer, and most of all the studio has produced some of the most technically brilliant animation.
http://www.youtube.com/watch?v=Tmcp4XNCWRY