This one is really hard, there are so many great illustrations in RPGs. There are also so many not-so-great illustrations that were the inspiration for memorable campaigns and characters from my youth. I am going to have to go with the cover of the very first gaming product I ever owned, Tolkien Quest: Night of the Nazgul.
You make think this is a cheat since this is a solo game book and not an RPG, but I bought this book because of this cover without even knowing what a game book really was. I wore this book and a few others in this line out, and then I used the rules from them to make up adventures for my friends to play. Eventually we started to question what this MERP thing was the book talked about and discovered that someone had already made a game that let you make up your own adventures. After failing to figure out MERP, we moved on to D&D.