Funny how you put that Itachi solo'd the Uchiha clan, when we all know that isn't true Lol
Nagato has killed more on screen, but Madara might've killed more off-screen. Sure, he grew up in a war period, but that doesn't mean he was going around getting finishing blows all the time. He's not the only Uchiha, plenty of fodders have gotten kills during his time.
Sasori probably has more, but it's hard to determine. We first have to take into account if he literally meant hundreds of thousands, or if it was just an exaggeration of like, a few hundred. Also, We have to take into account just how many people were in Konoha when Nagato destroyed it. Plus, it was said he was also going out to other nations, so that adds up more.