Aristoteles. If mankind would have accepted his brilliance earlier, we would have travelled to the moon in the 10th century instead of the 20th. No, don't say it: I'm definetly not exaggerating.
But then...what do I know? Here's the 'official' genius world-ranking:
http://www.wanderlist.com/genius

My personal favourite is currently ranked at #72, but I'm sure he'll claim the number 1 spot in no time.