The winner is no surprise. The only thing that matches Steve Purcell's cover art for LeChuck's Revenge ... is Steve Purcell's back cover art for LeChuck's Revenge.
Even though it's painted in oils to my knowledge, Purcell uses his perfected acrylic technique, building up color in washy layers. It was a truly daring concept to put the villain front and center. It's all about the eerie atmosphere for me. The only humorous element here is the dead bird on LeChuck's hat, and even that bird seems to be frozen in its last horrific seizures of agony. Also, I don't know if Purcell ever drew another monkey without a hat, pipe, eyepatch or at the very least a scimitar. The lighting is impressive, the composition is impressive, from the knowledgeable use of repoussoir to the usage of negative space (see how Guybrush is framed by the mast to the left and LeChuck's jacket to the right?).