I really dug the artwork for the original Al-Qadim sourcebook. It really fit the setting.
Tony Szczudlo's work on Birthright was fantastic. I especially like the big battle scene on the box cover and the booklets in the box; the entire thing is on the DM screen. Plus, the art in Blood Enemies for Birthright also fits the setting perfectly, especially that of Arnie Swekel.
Speaking of Arnie Swekel, his work on Pendragon really helped give that game and its supplements a distinct feel.
The art in the Amber Diceless RPG is also great. The trump pictures of all the major characters captured them really well and have helped solidify those characters in my mind when I re-read the Amber books.