I would be inclined to contribute that to CliffyB.

Nick by a landslide. He has an incredible mastery of poses, every single sprite frame is meticulously shaded, and he even drew countless sprites that didn't even get used in the game because he was so good they were no trouble to him. He's not limited to any particular shading style or sprite size and can convert sprites to different forms. Noogy had a couple good tilesets, but he just stole one of Nick's for a third of TSF, and the Lori sprites are pretty pathetic in that he drew her big, resized her, and made almost no effort to fix up the sprites at all. His strength is the bigger, cartoony drawings, but Nick beat him in that back in 1994.