Walt Disney and Co. When you watch Bambi and realized how much work went into that.... whew! Also some of his WWII work was cutting edge... and wouldn't be PC enough today.

Studio Ghibli (spelling?) - good animation with great stories.

Chuck Jones... The guy was a machine! BTW if you like his work, you have to track down a copy of Chuck Amuck. Great bio.

Brad Bird and Co. Incredibles! Iron Giant didn't get the box office hype but it was a very solid movie and good combo of 2D/3D

Who ever did the Triplets of Belvelle, great stylized look.

Frazetta, we'll not actually an animator, but the movie Fire and Ice based on his work had great look to it.

Square for pushing in Final Fantasy and more currently with Animatrix

And I definely have to agree with Groening and Pixar.