Einstein had Newton's work to go on. Newton INVENTED calculus, did he not? I could hardly understand it when I was taught it, but to invent it?!?! I think it would be a toss up between Newton and da Vinci. The problem is that they both contributed in such different areas that they complimented each other.
My personal vote goes to Newton. Without question. Nobody else even comes close.
http://www.metaweb.com/wiki/wiki.phtml?title=Isaac_Newton_(Alan_Sinder)
Liebniz would take exception to that.
I believe that Newton solved a previously unsolvable problem using antidifferential equations, thereby developing differential calculus. The Greeks actully started the whole mess by trying to solve the unsolvable, ruining many budding mathemeticians.