i was going to say minato as he could have almost single handily ended the 3rd great shinobi war. but than i remembered hashirama and madara were the most powerful shinobi of their generation, one of perpetual war between clans. then madara was revived and dropped two meators on a platoon of shinobi, so it's either madara or hashirama.
you can't compare some things.
on panel its madara. mainly from the meteor, but still.
off panel but still mentioned, kaguya has more kills then everyone on the list combined.
I would have to vote for Madara off this list. He lived in time before village/ninja system when it was constant fighting/war/killing contracts. He also lived a very long time (after the 3 ninja wars) so would have racked up a few kills in those years. He also created zetsu (which is a part of him) and they have been killing a lot of fodder shinobi's in this war. He also has racked up quite a few kills in a short time since his edo revival. I think Orochimaru would be 2nd on the list (mainly due to his test subjects).
I dunno, Man. You don't give "leave on sight" orders on just any shinobi. We've seen Minato casually merk around 50 ninjas. There's no telling how many he's killed.
The 3rd goes 1st though. fighting for days by yourself is crazy as hell.