It's controversial but I'm inclined to think it might be Deng Xiaoping. Holden and Rob on the 80k podcast:

Holden Karnofsky: Deng Xiaoping took over in China in the 1970s after Mao died. I feel like different leaders would have done different things, and he chose to go down the road of economic reforms that kicked off decades of unprecedented growth and poverty reduction.

Holden Karnofsky: I mean, that could be the most poverty reduction any individual has ever been responsible for. Especially if you look at what would have happened if he had somehow just not been around. He might be the person who’s had the most positive impact ever, to date. So I thought that was interesting.

Rob Wiblin: He was sent off to prison camps I think more than once when he fell out of favor with Mao, so it’s not that hard to imagine that he might not have been in the political scene.

Holden Karnofsky: Yeah, exactly.

Rob Wiblin: Or indeed may not have been on Earth, by that stage.

Holden Karnofsky: There’s probably a nearby world where just, things didn’t work out for Deng, they worked out for someone else, and everyone was so… Not everyone, but a lot of people were so much worse off. We don’t hear his name a lot, but gosh, I mean, what a person who made a big difference.

Viktor Zdanov

Estimate of lives saved: 15 million

This from Doing Good Better by William MacAskill

"In 1958, Zhdanov was a deputy minister of health for the Soviet Union. In May of that year, at the Eleventh World Health Assembly meeting in Minneapolis, Minnesota, during the Soviet Union’s first appearance in the assembly after a nine-year absence, Zhdanov described a visionary plan to eradicate smallpox. At the time, no disease had ever before been eradicated. No one knew if it could even be done. And no one expected such a suggestion to come from the USSR. But he conveyed his message with passion, conviction and optimism, boldly suggesting that the disease could be eradicated within ten years. Since smallpox was an exclusively human disease, he argued, it would be easier to eradicate than mosquito-borne infections such as malaria. He pointed to the Soviet Union’s success at eliminating smallpox, despite its vast territory and poor transportation networks. 
...
 By the force of his arguments, Zhdanov was successful. For the first time in its history, the WHO agreed to form a campaign to completely eradicate a disease. To assess how much good Zhdanov did, we should bear in mind that, even if he had not lobbied the WHO, smallpox would probably have been eradicated anyway. The problem was serious enough that someone would have started a campaign to fix it. Many of those 120 million lives that have been saved by smallpox eradication would therefore have been saved anyway. But there would probably have been a considerable delay in the smallpox eradication campaign. Suppose, therefore, that Zhdanov moved forward the eradication of smallpox by a decade. If so, then he alone prevented between 10 and 20 million deaths – about as much as if he’d achieved three decades of world peace.

Benjamin Lay. Probably did more than anyone else to kick off the abolitionist movement. There's a not-too-crazy story under which if not for him, slavery might still be common throughout the world today. (And under the same world model, the further rights advances/moral circle expansion that followed abolitionism – e.g., women's rights, gay rights, animal rights, etc – likely wouldn't have occurred either.)

Maurice Hilleman could have both been the one who has done the most good or the one who caused the most suffering. 

He developed over 40 vaccines, estimated to save 8 million lives / year [Biography].

But in a new documentary on him he's credited to have invented a vaccine for chickens, which caused to the price of chicken and eggs to drop dramatically (at 43 mins). "The economic impact of Marek's Disease was $2bn until, with a loss per hen of $15 or 60% of the total production value" [source]

There is a case to be made for Paul Ehrlich, the late 19th-century chemist recently highlighted by Cold Takes, who developed the staining techniques that allowed us to identify blood types, which enabled blood transfusions, which are estimated to have saved 1 Billion lives in this World Economic Forum article.

He shouldn't get credit for all the lives saved by blood transfusions, but it seems like he discovered a lot of important medical technology beyond the staining techniques (e.g. he started developing drugs that a target particular pathogen without affecting normal host cells, a novel enough concept that it got the fancy name "Zauberkugel", which translates to "magic bullet"). Check out Paul Ehrlich (1854-1915) and His Contributions to the Foundation and Birth of Translational Medicine if you want more details.

Great question and great answers so far. I always liked Alexander Fleming (discoverer of penicillin, which has certainly saved millions of lives) but I suspect someone else would have found it relatively quickly if he hadn't - although I've never looked into the details.

What about George Washington? He obviously wasn't the intellectual leader of the early USA and its form of government (and should only get a fraction of the credit / blame for everything that followed), but my sense (?) is that he played a key political (leadership) and military role in the revolution. Perhaps more importantly, he voluntarily stepped down after two terms as president, when he was still very popular. This seems rare in history (before or after his time) and likely led to more stable governance since then not only in the USA but also around the world. Arguably the US is the world's oldest continuous government and oldest democracy; the UK is tough to pin down, but I'd go with the Reform Act of 1832 if I had to pick a year when the House of Commons truly escaped from aristocratic influence.