Really interesting question Enron.
To discuss a couple of the suggestions so far:
Leo da Vinci was the true Renaissance man - gifted artist, skilled inventor. Certainly, he had tremendous vision, but I'd resist calling him the greatest mind ever.
Pastuer's discovery of pasteurisation was terribly important, but hardly stands out in the history of science and medicine.
Isaac Newton has a real claim to the title. He invented a chunk of modern mathematics called differential calculus. In fact, I think he's probably the single most important mathematician since Ancient Greece. His theory of gravity was truely revolutionary (don't dare imagine there's a pun in there!). He also did plenty more work on celestial mechanics. His laws of motion are the basis of classical physics.
I'd argue however for Einstein. His work still dominates modern physics. That so much of that quantum leap in technology in the last century has its roots in his work is stunning. His theorys of relativity are still being further tested, and still being backed up by emperial results. To add to that magnificent contribution to science, you've got his deep moral convictions. He campaigned against nuclear weapons, hated war.
Isaac Newton famously remarked in a letter to Robert Hooke, "If I have seen a little further it is by standing on the shoulders of Giants." When scientists today use that phrase, they mean people like Einstein and Newton. Either would be a good choice. Still, it is the centennial of Einstein's Annus Mirabilis.