All of them.

They're asking the questions, and doing the work to find the answers.

Linus Pauling. He won two Nobel prizes, the chemistry prize, and the peace prize. More personally, I read something he said years ago that galvanized my atheist beliefs. He was asked what he believed as an atheist, and he answered, "I believe all complicated phenomena can be explained using simpler scientific principles." I've been quoting him in response to that same question for the past 45 years, and it resonates with me just as much today as it did then.

Rachel Carlson. Because she warned us of our environments impending doom due to humanity's relentless pursuit of dominating the world.

Isaac Asimov, able to move from Teaching physics and writing textbooks to writing science fiction and back.

Bill Nye. I like humor with my science.

Nikola Tesla. Because he was arguably one of the smartest people that has ever lived. Because he pushed the world into the 21st century while he was alive. Because I feel a kinship with him, he was half crazy and all as well as brilliant.

Isaac Newton. He advanced physics and mathematics more than any other person in history.

Darwin, because he was a traveler, an adventurer, a scientist and a writer.

Sir David Attenborough (best Nature narrator), Stephen Hawking (one of the wisest), Neil deGrasse Tyson (love his space knowledge), Albert Einstein (my original inspiration.)

Emmy Noether of Noether's theorem who explained why energy is conserved in terms of underlying symmetries in nature. Her theorem is one of the most used in modern physics and she is rarely appreciated. The daughter of math professor in Germany, she was denied the ability to study math because women were not accepted at German universities. Nevertheless she attended classes and passed (aced) the exams -- eventually earning a doctorate in math. She was unable to get a position in the faculty but the members of the math dept realized her brilliance and let her teach their classes - but she was not paid by the university, rather her students paid her as a tutor. When she finally did get a job that paid, she had to flee Germany because this was the 1930s and she as also a Jew. She came to the US and taught at Bryn Mawr but unfortunately died shortly thereafter. Albert Einstein was one of her biggest fans and wrote a eulogy for her. I ran across her theorem repeatedly in physics classes but she was referred to only as E. Noether. I had no idea who this was until I ran across a fascinating article in the NY Times about the most important mathematician you never heard of. I make a point of mentioning her in my physics classes.

My daughter, because she's my daughter. One of her favorites is Stephen Hawking, she read him as a teen. I always liked that he would go on sitcoms and cartoons.

there are so many good ones, past and present, but my heart is torn between richard feynman and carl sagan.

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As much as people seem to despise him, I'm going to answer NDT.

No, he has not had the same impact on the world as many other famous scientists, but what he has done, to varying degrees of success, is make basic science more palatable to the average person. He has even made it "cool" to some, popularizing it beyond where it would otherwise be without him.

I'm not sure I'd be here if not for watching early NDT videos on YouTube. So that's my guy.

I have two. Both geniuses...
Carl Sagan and Richard Feynman.
Sagan because of his passion for the human race and our place in the cosmos.
Feynman for his unbounded curiosity of everything in this world, which I share. And for his wit, humor and charisma.

Dr. Bunsen Honeydew

Galileo. He bucked the ruling religion and spoke truth to power, though he was forced to recant later. Only Giordano Bruno was more influential and sacrificed more to bring the truth to the fore in the face of religious persecution.

I go for the ones where they had to make a leap so out of their and the world's prior beliefs that it caused a complete change in our world view. This means not just a cool discovery but the discovery (or rediscovery) was made in a state of mind that was against such an idea. To me that's a mark of a great scientist. They had to accept someyhing that completely upsets and rewrites their reality. There was no lead up....it completely blindsided scientists and the world. Even among the great scientists, only a few have done this:

Copernicus - we go around the Sun. (We ain't the Center of the Universe.)

Darwin - new species originated from prior species through random modification and survival of the fittest. (We ain't specially created by God.)

Newton - The mathematical rule of gravity that applies to an apple also applies to the moon and planets. (The same exact rules that apply to earth objects applies to heavenly objects....there are no heavenly objects!)

Planck - energy at the atomic scale can only have certain values, i.e. energy is quantized. (Physical quantities cannot have certain values atbthe atomic scale!)

Einstein: 1) because the speed of light is constant, any things (atoms) travelling near the speed of light but at different speeds won't experience the same event sequence. (There is no such thing as simultaneous events, simultaniety is an illusion...therefore universal time does not exist.)
and 2) mass or energy warps space-time to create gravity (mass and energy have the same effect thus have an equivalence; space and time are physical quantities; space and time are not separable)

Hubble: the star farther away are moving away from us. (The universe isnt static it's static its expanding).

DeBroglie: yeah we've known light is both a particle and a wave....but so is all of matter! (Everything is a "wave-particle" )

Heisenberg: there are certain pairs of quantities that when measuring one with great precision reduces precision measurement in the other. In fact there is a limit to how precise we can measure anything. (we cannot measure anything with arbitrary precision.)

Schroedinger: the act of observing affects the outcome of a quantum (atomic) system...but each time we commit an act of obervation the outcome is different, based on the different probabilities of possible states the quantum system is allowed to be in. (Prior to being disturbed by observation, a quantum system seems to exist in all of its allowed states at once!)

The 1920's were the last of the single scientist-world changer.

The most recent (1998) world changer was done by a team of scientists showing that the universe is not just expanding, but the expansion is accelerating.

Note I did not include discoveries that had a slow gentle reveal that involved decades or centuries and many scientist, because often the scientist just had to take the natural next step without fighting any internalized world view.

Let me know which ones I've missed.

Anyways, for me, Each one had to break out of centuries of ingrained thought and push themselves into a new way of thinking. So they are all my favorites!

Even among these, I think Einstein stands out. He was able to find the far reaching universe rewriting consequences out a simple fact simply by thinking about it. Twice! And he has had so many other contributions, each would make any scientist's career. It's astonishing how much he accomplished.

We will likely never see another like him for atleast a century or two, maybe a millennia.

Hypatia as she had no fear of speaking her mind in an age of much darkness and Carl Sagan for his poetic style and making science romantic.

Carl Sagan is amongst my favorite, most likely number one. I have read his book Cosmos three times and love it and learned something new every time I read it. I don’t recall watching his series “Cosmos” but I have watched Neil de Grasse Tyson’s version and I thought it was excellent. Simply put, so the layman can enter the world of astrophysics.

Carl Sagan. Steven Hawking. Neil DeGrasse Tyson. Nichola Tesla.

Historically there are scores of them...but two recent ones have to be Stephen Hawking and Sir David Attenborough.. although the latter is strictly speaking not a scientist, but a naturalist, and quite an amazing human being, Stephen Hawking for his A Brief History Of Time amongst all his other publications...and his dignity and endurance in overcoming his motor neurone disease. Both are/were atheists.

Collectively all scientists contributed. Besides many which have already been mentioned on here including some of my favourites I thought this write - up of Indian scientists is worth the read.

[thebetterindia.com]

Richard Dawkins - I discovered him and his most popular book, The Selfish Gene, in college. He introduced the (scientific) concept of the meme which internet memes are named after. His idea of both genes and mental constructs as virus-like replicators has so much potential I think.

There are two that have changed our lives in such a profound way.
A) Interesting no one have mentioned Albert yet.
B) Shockley opened the door of electronics to unfold the technology that have us all sharing thoughts today through digital computers, which paved the way for the internet and pretty much everything we know today. He invented the transistor.

Carl Sagan... Because he really brought science to the masses. I read all his books and watched every program he was on. I miss him! My new favorite is Neil deGrasse Tyson for similar reasons plus he steps into the political realm every now and then.