I consider Erol Otus to be the best thing that ever happened to TSR's art department, but several others are worthy of mention. Like Roslof. Dig these images from the old AD&D Monster Cards:
Here are two of the wimpiest monsters in the game, a goblin and a kobold, and Roslof does a great job making them scary. I'm going to go out on a limb and say that's the best kobold I've ever seen. The grin, the scales, the claws, the eyes! And I love Roslof's use of color to give weapons and armor a sheen.