Carl Sagan because of how could word science to the level of most people and also inspire some to pursue learning more!

Roger Bacon a 13th century Franciscan friar who promoted the scientific method approach to science. Collecting facts and experimenting before postulating a new idea or theorem, therefore changing attitudes in scholars of the time.

Richard Feynman - Loved his books.

i don't know any really but i guess many of them are good as they have to work on facts

Difficult query: I think Andrei Sakharov as he was vital to the dissident movement in the USSR and became a true humanist, aghast at his own work.

The 'Three Karls' influenced my thinking: Clausewitz, Marx, and Darwin.

Darwin and Wallace (separately of course) Their ability to use their powers of observation and take disparate pieces of information and realize their significance. From these surficially unrelated observations an explanation for how life operates evolved (pun intended). Nineternth Century preconceptions and biases made their conclusions even more heroic.

Darwin never totally abandoned religion and a belief in a God (a product of his age) . But the fact that we are debating their ideas 190 years later suggests the significance of thier contributions. No other area of science is so hotly debated by the lay public. Despite the fact that their conclusions have been thoroughly proven over and over again.

Carl Sagan, for giving us masterpieces like Cosmos and Contact.

Batman... for obvious reasons. lol

Carl Sagan. I saw his program Cosmos, and it saved me. I liked science, but I was not an avid student of it. Moreover, I was hanging with some friends who were born again. After watching the program I bid good bye to my friends and started to learn science in earnest. I would recommend a book titled The God Delusion by Richard Dawkins. It's really, really good.

Marion my love you are confused Sir Richard Attenborough, Now Baron Attenborough is an actor and film director. Brother of Sir David Attenborough the naturalist

The scientist who is frequently searching for answers and shares what they learn with humility and clarity.

My uncle who worked with CERN and is also known as the “Father of String Theory”. Great man!

Nikola Tesla fascinating man with a fascinating life, a dreamer and innovator.

Einstein and Shockley.

Thomas Young
The last human that was able to study and contribute every aspect of natural sciences available at time.

By far . . . . Thomas Henry Huxley. I have his portrait hanging on my wall, and consider him to be one of my favorite historical figures . . .

He was known also by the name "Darwin's Bulldog", for his defense of Darwin's Theory of Evolution . . . He was a biologist who learned most of what he knew through self-education. At the age of 35, Thomas Henry Huxley, in 1860, debated the Bishop Samuel Wilberforce, 55, who in the debate asked Huxley if it was through his grandfather or his grandmother that he claimed his descent from a monkey.
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Huxley replied that he would not be ashamed to have a monkey for his ancestor, but he would be ashamed to be related to a man who used his great gifts to obscure the truth.
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Huxley was often known to, in the course of his many debates, quietly state that he knew nothing about the supernatural which his opponents claimed to have firm belief in . . . . . then, somewhat louder, add . . . "And neither do you."

It would have to be my namesake, Lord Kelvin. Although he was a bit of a religious nut and he never got the hang of nuclear physics. He was the last of the great 19th century scientists. Publishing over 200 papers before the age of 21, some under assumed names in order not to embarrass his tutors. His early work on superconductivity earned his name on the SI unit of temperature. His work on underwater communications (he helped design the 1st trans-Atlantic phone cable) led to my dad who was a radio guy in a submarine in WW2 naming me after him.

Lots of good ones here already. I don't have a favorite, but I want to mention a couple that I think will be left out of most nomination lists, and so may be finds for others to consider:

Giordano Bruno
Rosalind Franklin

also:
Darwin, Einstein, Kepler, Galileo, the Curies and Feynman would have to be in there somewhere, for my own considerations, and from what little I know.

None, just like I don't have a favorite prostitute. When I want heroes I look into sports and drop them too. Maybe one day a Revolutionary comes along and cater my fancy while surviving capitalism and then again... maybe not. I will let "scientologists" do their thing with science while inflating their egos. science the new Religion.

Clair Campbell Patterson. Pioneered radioisotopic dating, created the first 'clean room' lab, dated the earth at 4.55 billion years, a date that has only been refined by a few percent since.
If that wasn't enough, when he noticed that his samples were being constantly contaminated with atmospheric lead, he tirelessly campaigned against the use of lead additives in petrol in the face of a concerted smear and intimidation campaign fought by the chemical and automotive industries.

Most likely Carl Sagan. I first got a copy of his Demon Haunted World back when I still believed in spirits and found out the book was scientific and really different. I could hardly put it down and I'm convinced that Sagan had some hidden influence with me when I finally came out as atheist. I saw him as a scientist who made sense.

It’s difficult to answer because it depends on the metric you are judging. Most valuable to humanity? Well that could be debated eternally without arriving at a conclusion. Smartest? Again it is really impossible to say as many work based on the triumphs of their predecessors and in completely different fields.

I’m going to go with most inspiring for my metric and with that in mind I love the image of someone like Archimedes problem solving and just staring down a circle on paper and attacking the problem of figuring out its area. Then taking it a step further to do the same with a sphere. I could easily have chosen Pythagoras or several others for the same reasons.

I like that imagery because while these figures made achievements equivalent to modern scientific marvels, being that they are so fundamental, they are problems I can imagine facing myself and tackling. Any person of the day could have decided to sit down and figure them out. I can’t say the same about someone inventing the microchip or decoding DNA.

There are many great scientists, but your question was "favorite" scientist. My favorite scientists are those who agree with me that the universe is 100% natural with nothing supernatural about it.

Brian Cox, and probably because of the chemistry he has with his audiences.