Charles Darwin's work On the Origin of Species is celebrating its 15oth birthday/publication-day today. A lot of news outlets are talking about the influence of this book on our scientific thinking about evolution. Checkout NPR, BBC and NYTimes for more info. I really don't have much more to add to that information-wise about Darwin. I've read parts of On the Origin and as a thinker he was way ahead of his time with some really great thoughts.
But honestly, just between you, me, and the blogosphere he is not my favorite historic (aka. dead) scientist. Beating him out by a long shot is Wilhelm Hofmeister. Ok, so you have probably never heard of Hofmeister. His position as an unknown underdog is one of the reasons I like him and his scientific discoveries. I study mosses. I tend to like the underdogs and migrate toward championing them.
Hofmeister's major discovery was to observe and outline the alternation of generations in many different kinds of plants from bryophytes to flowering plants. Basically he figured out how different parts of the plant life cycle go from gametophyte to sporophyte and then back again. He was the first scientist to figure out and discribe this important plant phenomenon.
He only had a basic education equivelent to trade school through age 15 and was entirely a self-taught botanist. Also he was very near-sighted. So much so that he sometimes did not recognize people walking down the street. However this sort of turned his eyes into magnifying lenses enabling him to see tiny plant parts and mini mosses.
Check out this scientific journal article to read more about Hofmeister and his scientific contributions.
The Genius of Wilhelm Hofmeister: The Origin of Causal-Analytical Research in Plant Development. Donald R. Kaplan and Todd J. Cooke. American Journal of Botany, Vol. 83, No. 12 (Dec., 1996), pp. 1647-1660.
Hofmeister also has a well-referenced wiki entry that appears to be accurate considering its wiki-ness.