Georg Cantor created the subject of set theory, and he gets full credit for this.  But (1)  there are important set-theoretic principles that Cantor was the first to assert, but for which others get the credit, (2) There are theorems that Cantor was the first to prove, but  which are attributed to others, and (3) Cantor is not sufficiently appreciated as a philosopher of mathematics.  If Cantor's concepts and arguments had been generally understood, the history of set theory would have been different. Admittedly his being under-appreciated was, to a large extent, his own fault.