My favorite Garfield television special has to be 1988's Garfield: His 9 Lives.
Available on the DVD "Garfield Fantasies", the special is very unique like the book it's based on.
Different artists contributed to the book illustrating Garfield in their own way.
The special mimics this by using various animation styles.
My favorite segment is Life # 4: The Court Musician. It's cartoony and has a "design-y" feel to it.