Aizen was the first to create a Hogyoku, though (no, it wasn't Urahara). He also is responsible for the experiments leading to hollowification and arrancarization with even specialized goals like Wonderweis (that he kept in secret for a century). He also created artificial hollows that could conceal their reiatsu and were used for surveillance, and the one that could delete zanpakutos.
One thing's for sure though, Mayuri, Urahara, and Aizen are the smartest beings there, and the only clear lead is Urahara. And even Mayuri sometimes does some things better than him (like the reiatsu amplifier or Aizen's chair).