Sasori.
C'mon, he killed the strongest Kazekage when he was like a teenager. Orochimaru killed Gaara's dad, and he seemed to have relied on Kabuto to get that done.
Orochimaru couldn't take down a village. Sasori took down a NATION.
Sasori's attitude and swag is just through the roof. When he heard Sasuke killed Orochimaru, Sasori was actually kind of infuriated and depressed because he wanted to do it with his own two hands. Having such confidence says something about a character. And the fact that Deidara calls Sasori "Master", speaks volumes about Sasori. He is not one to mess around with!
The thing that makes Orochimaru so powerful is his durability. But I think it can only go so far. Sure he can regenerate from almost ANY attack, but it's still undecided on whether poison will affect him. If it does, only one SCRATCH will do the trick. How many Kage level ninjas fight without getting ONE scratch. Almost none. Orochimaru has some pretty powerful jutsu, and I'm sure he has a large scale one that can destroy most of Sasori's puppets.
But I think Sasori never really went all out. He seemed always indifferent when fighting. I'm sure there is another level to him where he REALLY goes all out. Didn't he also die with one more scroll on his back? Maybe that was an even more powerful trump card. Also, there is still speculation about how chakra strings work. We know puppeteers can connect them to OTHER HUMANS (ie. Chiyo and Sakura). For all we know, Sasori can connect his strings to the back of Orochimaru and make him stab himself with a kunai over and over again. We also know a puppeteer can connect chakra strings to inanimate OBJECTS. Sasori can steal Orochimaru's Kusangani just by attaching a chakra string to it and yanking it out of his hand! Then he'll give it to one of his puppets!
Plus, I truly believe Sasori's ultimate attack is to have 100+ various jutsus fire at you at once. Since you can have over 100 puppets out at once, and they all can maintain their original abilities, imagine the damage he could do! You think Kakuzu is impressive by firing 2 or 3 elemental attacks at once? IMAGINE SASORI FIRING 100+ elemental jutsu at once! WATER, FIRE, WIND, LIGHTNING, EARTH...you name it! And that my friends, is how you take down a NATION!