I would say Oro but now when I think about it his L record is to high.
three sannins vs Hanzo really L but Tie
Oro vs Hiruzen really W but really old timer
Oro vs Itach L
Oro vs Itach with Kabuto body L
Oro vs Sasuke (even if he was sick) L
Oro vs Itachi in Sasuke boddy L
Oro and Kabuto vs Naruto , Jman ,**** princess and assistant L or Tie because they retreated
Oro and Kabuto vs Sai , Yamato, Sakura, Naruto
So call he became the final villain and lost again...
Now Madara he keeps getting soloed by the 1 Hokage
not once has he won.
Basically, if their was suppose to be an ending villain let it be Oro-Man (both combined)