The millenium seemed to spur a lot of people to compile "Top 100" or "Best 100" lists of many things, including movies (by the American Film Institute) and books (by the Modern Library). Mathematicians were not immune, and at a mathematics conference in July, 1999, Paul and Jack Abad presented their list of "The Hundred Greatest Theorems." Their ranking is based on the following criteria: "the place the theorem holds in the literature, the quality of the proof, and the unexpectedness of the result."
A. Fauzan, D. Slettenhaar, and T. Plomp. Proceedings of the 3 rdInternational Mathematics Education and Society Conference. Copenhagen, Denmark: Center for Research in Learning Mathematics, (2002)