The Incredible Hulk #1 (May 1962)

Look at the skin: the Hulk debuted gray, and he only turned green from the second issue on. Kirby stacks the monster behind a stunned Bruce Banner and asks "Is he man or monster or... is he both?" It is the clearest single image of the Jekyll-and-Hyde idea behind the character, and one of the most expensive Silver Age keys on the market.