Whirly has given this a lot of thought -
Archimedes is probably the greatest of the ancient world
Leonardo Da Vinci the greatest of the renaissance
Newton the greatest post renaissance pre 20th century
The last century is very hard cases could be made for all the obvios ones Bohr, Heisenberg, Einstein, Watson and Crick, etc, etc. Some would say the most naturally gifted was the maths mystic Srinivasa Ramanujan, I am going to go with a man who like Newton, Archimedes and Da Vinci was an all rounder Richard Feynman.
Who? you say 🙂
Richard P. Feynman, scientist, teacher, raconteur, and musician._ He assisted in the development of the atomic bomb, expanded the understanding of quantum electrodynamics, translated Mayan hieroglyphics, and cut to the heart of the Challenger disaster.
He won the Nobel prize for Physics and his Genius is only now starting to be truly apprciated - In a time of giants he was the 20th Centuries Da Vinci
http://www.amasci.com/feynman.html
"I was born not knowing and have only had a little time to change that here and there."
Richard P. Feynman 1918 - 1988