Rembrandt van Rijn. Above is a self portrait done when he was about 28. Note the brilliant (in both senses) use of illumination on his face and collar, the almost palpable fleshiness of his face and texture of his garment, and the intensity of his half-shaded eyes.
Addendum: Michael Sorgatz tells me they've just rehung the Rembrandt gallery at the Metropolitan Museum of Art, and that it's "fantastic."
Images: Self-Portrait, Wikipaintings; The Mill, Wikipedia.