That’s a good question (unlike so many other questions at Fluther). To be the greatest, I think the character ought to:
• Be a golden ager. I think of Warner in the ‘30s and ‘40s as the best period for cartoons.
• Be funny. Which rules out all but a few from Disney.
• Be part of a good ensemble. I might’ve gone for Felix the Cat, if his co-stars in his later pictures weren’t so lame.
• Be well animated. Sorry, but that strikes out characters from every studio that ever saved a buck with limited animation.
So my vote goes for Roger Rabbit, who worked for Maroon Cartoons in the ‘30s, is roll-in-the-aisles funny, works harmoniously with Jessica Rabbit and Baby Herman, and moves on screen with grace and originality and smooth lines.