Orochimaru is a good candidate too, he abducted alot of people to xperience on, like Yamato's case where he kidnapped 60 baby's and yamato was only survivor.
Madara stomped entire division plus during age of endless wars he would have killed many.
Obito is second he killed many while retreiving g and s brothers plus with jyubi and mokuton.
Pain dont know minato probably during the 3rd war