John Lothropp: Ancestor of 80k people including 6 US presidents (en.wikipedia.org)

A strong contender! https://xkcd.com/1545/

He died in 1653. Assuming 2 children per generation every 20 years, that's like 18 generations, and the expected mumber of decendants is 2^18 = 262144 = 262k (or should I use 2^19-1).