Studies For The Libyan Sibyl of the Sistine Chapel by Michelangelo, circa 1510-11. It is only 11 3/8 x 8 7/16 inches and is simply marvelous. I'm not the only one as evidenced by the fact that people, generation after generation, have looked at it and decided to go to the trouble to keep a fragile sheet of paper safe and intact for over 500 years.
The use and the economy of line is extraordinary. Yet it was intended as a working drawing with scribbles and splatters of ink. The varying states of completion of various elements makes it a lesson in drawing for those with the talent to learn from it.
I should mention that the back of the sheet, which is also a study for the Libyan Sibil, is much less impressive. The Libyan Sibil as painted on the Sistine Chapel lacks much of the charm of the drawing although who doesn't like a giant book. (here is a zoomable view of the Sistine Chapel)
A sibyl is any of certain women of antiquity reputed to possess powers of prophecy or divination. (The Libyan Sibyl is a woman but I think this drawing is clearly of a man.)