John Snow

For finally convincing westerners that microbes exist. Which got the ball rolling on like, actual medicine.

That's all.

Louis Pasteur?

https://en.m.wikipedia.org/wiki/Louis_Pasteur

Johannes Gutenberg, the inventor of the printing press.

Fritz Haber, the Veritasium video about him is fascinating (The Man who Killed Milioms and Saved Bilions). He developed the chemical process to efficiently synthesize ammonia, one of the key discoveries that allowed mass adoption of fertilizers and the incredibly rapid growth of the human population in the 20th century (you could say that thanks to him, bilions of people could live and be fed by modern agriculture).

Tragically, he also had a fundamental role in developing chemical weapons during WWI, although he belived their use would reduce the number of deaths as army would simply avoid gassed zones, so who knows if he really intended and believed in the milions of deaths he caused. Ironically, he also helped developing Zyklon B during the rise of nazism (while it was still used as a pesticide), but was quickly forced to flee from Germany because of jewish origin. Later, his last invention would be used to kill even more people.

Norman borlaug and Fritz Haber. The first was basically the father of modern agriculture helping feed over a billion people. The latter known as the man that saved billions and killed millions, helped develop the haber bosch process that produces ammonia used in fertillizers that are responsible for feeding half the world's population. It was also used in explosives hence the "killed millions" part.

Louis Pasteur

Politicians and kings rarely do something they weren't forced to, and inventors are rarely without competition, so I take issue with most of the responses here.

Instead, I'll go with naval officer Vasily Arkhipov, who, if he had decided to agree with the normal officers of the submarine he happened to be on, would have started a hot Cold War on 27 October, 1962.

Then again, there was a separate, slightly less severe close call the same day, so if you butterfly that who knows what else happens. It was a crazy time where few understood nuclear diplomacy and cold warfare, but nukes were ubiquitous, and were being treated like normal weapons. We got lucky.

Norman Borlaug. His agricultural innovations have saved literal billions of of lives from starvation and malnutrition.

Linus Torvalds or Richard Stallman

Who ever started the whole enlightenment thing, with the idea that there is no god and we are responsible for our self.

Norman Borlaug helped develop a lot of techniques used by developing nations to gain food self-sufficency.

The Mitochondrial Eve.

Mathematicians, Physicists, Scientists, and Astronomers: Good effort everyone. The foundation of a rational world.

Very Notable Mentions:

• Chemist: Fritz Haber. 1/3 of world food production today can be attributed to his discovery. Also an enormous negative impact, see German Chemical Warfare.

• Biologist: Gregor Mendel. Monk who discovered the basis for genetics.

• Ecologist: Charles Darwin. Discovered the theory of evolution.

• Philosopher: Socrates. Critical Thinking.

• Computers: Charles Babbage, Ada Lovelace, and Alan Turing. See empowerment of computation and relegating ridiculously complex math and data collection to machines.

• Computer Networking: J. C. R. Licklider, DARPA, and Tim Berners-Lee. See Internet and I/O on a global scale. Both positive and negative.

• Finally, the largest net positive of all: Artists. Yes, artists. Popularity as the prime determinant by nature of their work. For inspiration, desire, meaning, peace, community, and emotion. The language of all, an instinctive form of communication.

My visual pick is Leonardo da Vinci as both a practical and artistic contributor. As for classical, it's nearly impossible to pick, but I'd say Beethoven and then Bach.

Whoever first domesticated fire. Whatever his name was, I forgot.

-Sir Alexander Fleming (guy who discovered the anti-biotic properties of penicillin)

-Sir Isaac Newton or alternatively, Gottfried Leibniz (they both independently of one another invented Calculus roughly around the same time)

-Bill Watterson

James Clerk Maxwell. If it uses electricity then it's based on Maxwell's equations.

Dolly Parton

Bell Labs and the invention of the transistor.

my mother

Lenin.

The person who figured out how to make fire