Who's the greatest artist of the 20th century?

Picasso was by far the greatest artist of the 20th century. How to proof this ? An economist recently demonstrated that if you round up every art history textbook of the past 15 years and see whose art is reproduced most frequently, then Picasso, with 395 illustrations in 33 textbooks, scores nearly as many as his three closest rivals (Matisse, Duchamp, and Mondrian) put together.