That's really hard to say as smartness is ultimately subjective and hard to define. Most people use I.Q. as a measure of smartness, but the exam only tests a persons general ability to understand concepts as compared to the general population. So, if you're idea of smart is knowing a lot of things, someone could have a very low I.Q. and still be very smart.
Steven Hawkins has been deemed smartest man. Google him and you'll be amazed at his physical condition. I understand they have taken some of his DNA and its on the Space Station. His isn't the only DNA, but it's for future humans if earth is destroyed. You can probably read about this too at Wikepedia.