Hans Rott. Sadly he went completely mad pretty early on and died young in a mental institution.
Mahler was Rott's roommate as a student in Vienna for a while, and Mahler kind of tried to copy and develop his style further. Some say he was a hoax; that Mahler made him up. But Rott's genius was far above Mahler's level. I simply can't believe that Mahler would ascribe his best works to someone else.