Going to have to throw my vote behind Aerong Damphair (VM), although there are plenty of great cards to choose from (especially in that set!), including some really nice landscapes in the unpronouncable Martell locations. The artwork is very good, but the pose is inspired and I think does a phenomenal job of capturing the character. I'd like to give an honorouable mention to Seductive promise, for similar reasons - an unusual but excellent choice to represent the card name.
As for least favourites, "The Art of Diplomacy" bothers me. The artwork is lousy, and there were so many other good choices of representations for a card of that name.