Darth Bane hands down. He was the Sithari, "The One", even after he got rid of his "armor". He later learned how to swap souls with anyone.
Darth Zanah (sp?) did not defeat Bane, he transferred his essence into her body. She was young and healthy, but not worthy of taking his place as master.
You are wrong. Zannah officially defeated Bane.
Mace Windu would probably beat Darth Bane if we take into consideration the rule of two.