...according to an article in Slate. Who's the Greatest Artist of the 20th Century? says that David Galenson, an economist, has proved that Picasso is the greatest artist of the last century because his work has been reproduced the most in art history textbooks of the last 15 years:
Picasso, with 395 illustrations in 33 textbooks, scores nearly as many as his three closest rivals (Matisse, Duchamp, and Mondrian) put together.
The article goes on to say:
...the counting is the start, not the end. What really interests Galenson is when artists reach their peak. His method allows him to show, for example, that the artists who made conceptual leaps (Johns, Picasso, Duchamp, Warhol) peaked far earlier than so-called "experimental" artists for whom practice made perfect (Kandinsky, Rothko, Mondrian). Johns was 25 when he produced his most-reproduced pieces. Mondrian was 71.
The rest of the article - about how the same sort of counting is used in Google search algorithms and other areas - is also interesting.
I do wonder why he only looked at the textbooks of the last fifteen years. The last five would make more sense if you wanted a post-20th century view. Or the last twenty-five to forty would make sense if you wanted a slightly broader range of opinion (art having trends just as everything else does).