historically - Kafelnikov, Davydenko, Chesnokov, Cherkasov. Not his best surface, but Safin could certainly play well on clay. On a lower level, Golubev and Gabashvili posted good results on the dirt. Khachanov has some good results on clay, though has done better generally on hardcourts (aside from the French QF). Same with (An.) Kuznetsov.