I still have to vote for Gaudio as the weakest.
Fun fact: Gaudio never even made the quarterfinals in a major other than that FO he won.
I can't find any other major winner who also only had one QF or better showing at a major in his career. Johansson at least made the USO QFs twice and the Wimbledon semis once. Korda made the QFs of a major 6 different times in his career.