Artfacts.net has introduced a system for ranking artists using a points-system based on their exhibition history. The system is explained here. The top 3 are (currently) Picasso, Warhol, and Bruce Nauman. And here's a piece about the site from the UK Telegraph. Relatedly, a University of Chicago economist recently "proved" that Picasso was the greatest artist of the 20th century by a similar method: he looked at every art history textbook of the past 15 years and simply counted up the number of reproductions of each artist's works.