King Edward III

We are direct descendants of King Edward III of England and his wife, Philippa of Hainault, through their son, John of Gaunt, 1st Duke of Lancaster.  Edward was born at Windsor Castle in 1312.  He became King at age 14 (1327) and reigned until his death in 1377.  He created the Duchy of Cornwall and founded the Order of the Garter.  He was a good king and an excellent military leader who led England into the Hundred Years War with France.

This might seem like an impressive claim... however, Edward had twelve children with Philippa, plus at least three more by his mistress Alice Perrers.  A genealogist from the UK calculated the probabilities and came to this conclusion:
There is an extremely high probability that a modern English person with predominantly English ancestry descends from Edward III, at a very minimum over 99%, and more likely very close to 100%. The number of descendants of Edward III must therefore include nearly all of the population of England, and probably much of the populations of the rest of the UK and Eire, as well as many millions in the USA, former British colonies and Europe, so 100 million seems a conservative estimate. 

The special thing about this claim is that I have traced our ancestry back to England in the 1200's through my father's maternal grandmother, Georgia Powell.  Her ninth great grandfather Judd married Katherine Norwood whose maternal line goes back another nine generations to Edward III.