From what's been observed in the manga, it'd be Nagato.
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He killed the vast majority of Konhagakure's residents and single-handedly ended Amegakure's previous regime.
He was reputed to have killed everyone ever associated with Hanzō (family, friends, and clients).
Nagato also has at least eight other confirmed kills and of course there's all the battles he's never lost. (Konan hype)