Joe Mathieu was one of my all-time favorite Sesame Street artists as a kid. Many of the Sesame Street Random House Pictureback books I had as a kid (the "A Visit to the Sesame Street..." series, "Grover and the Everything in the Whole Wide World Museum," "We're Different, We're the Same," "A My Name is Alice" and "Sesame Street Stays Up Late") were all illustrated by Joe Mathieu. I also enjoyed his work in "The Exciting Adventures of Super Grover" and "The Sesame Street Dictionary." When it comes to the styles he does, I like the style he did in the 1980s and 1990s best, like here:
http://images2.wikia.nocookie.net/muppet/images/9/99/Book.weredifferentsame.jpg
I think his and Tom Cooke's styles are the most distinctive.