My list is dominated by scientists, as I'm a science geek, and history of science is the history I love best.
1. Charles Darwin (Biology, Geology, Developmental psychology - the quintesential scientist, best known for being the first to explain and persuasively argue for a physical mechanism by which life diversifies into an unlimited variety of species, form initially simple forms. The theory of evolution has of course evolved since the time of Darwin, but still forms the core of what is known as the modern synthesis, with most of his insights having received confirmation, and overwhelming evidentiary support since the idea was first proposed. As a total biology geek, he's pretty much my scientific hero.)
2. Galileo Galilei (Natural philosopher, mathematician and astronomer - one of the founders and key early proponents of the scientific method, and a member of the first scientifc society.)
3. Isaac Newton (For his groundbreaking work in physics, and invention of Calculus - though it should be mentioned that Leibniz independently came up with the same mathematical ideas at the same time. Not a likeable character at all, but undeniably brilliant.)
4. David Hume (One of my favorite philospher, an empiricist and enlightenment thinker. In his "Dialogues Concerning Natural Religion" he quite robustly challenges the argument from design, at a time before Darwin, and thus without a benefit of being able to account for the appearance of design. It is a brilliant read, one that I highly recommend.)
5. Roobert Hooke (Newton's contemporary, and rival. Newton was not an easy person to get along with, and he could REALLY hold a grudge. Newton published something that had obviously drawn from Hooke's earlier work, without giving proper attribution of credit to Hooke. Hooke was understandably dismayed, and when this objection came to Isaac's ears, he in turn was thoroughly offended. THIS dispute is actually the source of the famous line "If I have seen further, it is because I stand on the shoulders of giants", and rather than the humble appreciation of previous scientists that it's often taken to be, it was probably a slight directed at Hooke. You see, Hooke was a diminuative man, with poor constitution and a crooked back - pretty much the opposite of a "giant". What Newton intended was to add insult to injury by giving credit to other, earlier scientists, but slyly excluding Hooke. Something which would not have been missed by the scientific community at the time, aware of the Newton-Hooke dispute. Hooke was a very significant contributor to science, for example, for his work with the study of the microscopic world, and he was the most active member of the Royal Society, but after his death, Newton did his best to erase him from all records, to diminish his imprint on history. THAT is how far Newton could take a grudge.)
6. Albert Einstein (Needs no introduction.)
7. Ada Lovelace (The daughter of the famous poet Lord George Byron. A Brilliant mathematician and a woman working in a male dominated field in the 19th century. She was way ahead of her time, in foreseeing in Charles Babbage's "difference engine", the potential of making a computer that could do everything from making music to creating graphics. For this astonishing insight she's sometimes called "the world's first computer programmer", and the programming language "Ada" was named after her.
8. Richard Feynman (The person of the most recent history, an extraordinary physicist, who's most significant scientific contribution is probably Feynman diagrams, that provided a way for scientists to understand and interpret the very complex behavior of subatomic particles, by translating convoluted mathematical expressions into fairly simple pictorial forms. This is what makes it possible for people to understand subatomic physics, that would go above the heads of 99.999999% of people otherwise. But what he reallly excelled at was explaining scientific concepts at the level of his audience, whatever the level of the audience was, and doing it in an exiting way that conveyed his enthusiasm for science, and allowed his audience to experience some of that beauty. Recommend looking him up on YouTube.
Here's an example.)
9. Bertrand Russell (A Philosopher, a profoundly intelligent and multi-tallented man who's curiosity knew no bounds, and a great source of pitthy quotes: "If fifty million people say a foolish thing, it's still a foolish thing.")
10. Edmond Halley (Astronomer, Newton's contemporary, best known for calculating the orbit of Halley's comet, also was first to show that stars, that were thought to be "fixed" in the sky, had actually moved significantly, by comparing his measurements with those of Ptolemy some 1800 years earlier. He was also a pioneer in attempting to use scientific methods to date ancient monuments - he tried to date the Stonehedge, and altough he got the date wrong, the idea of using science to date things persisted and developed.)
I probably forgot someone, that, on further reflection would have displaced someone else on the list.