Why and how was Leonardo Di Vinci the greatest painter of his time?

He is considered among the greatest painters and men of knowledge for many reasons. In speaking of art alone, he was considered to be the greatest by many incluiding Raphael, Michelangelo was an exception and there was a rivalry between the two. You have to hold a sufficient knowledge of art in order to completely grasp why. To put it in simplest terms he was a really good artist, and it was said during his time that a fower painted by Leonardo was better than one made by nature. Leonardo didn't simply reproduce nature and thought of artists who did this mediocre. Leonardo studied and yzed his subjects to the miniscule detail, for him it was important for the artist to have an understanding of nature in order to reproduce it. If you read his notebooks( if you look on the net you can find them), he writes about every subject imaginable at the time architecture, botany, engineering, anatomy, medicine, flight, optics, mechanics, hydraulics, mathematics, geology etc... By understanding this subjects, leonardo believed he could better reproduce nature, although leonardo didn't study all of these fields just to paint, he did apply them in his paintings. Which is why when you see an angel painted by leonardo for instance, the wings are anatomically correct to those of birds. His method of shading(sfumato) is flawless, and he achieved this by his understanding of optics. The list goes on and on, simply read his entry on wikipedia and follow the links. Or also search Bulent Atalay there are some videos on the net(try google video) where he talks about Leonardo, you might find other if you search.