I read somewhere a while back that Elizabeth II of England had the oldest verifiable family tree, tracing her ancestry back to a 6th century AD Saxon king.
I remember thinking at the time, what about the Chinese? Combine Confucian reverence and worship for ancestors and record keeping stretching way back before the Christian era, surely that adds up to verifiable genealogies that would knock Liz off her perch. (Or presumed perch, the source I read may well have been wrong).
Last time I looked at the Guinness World Records stuff, they had your answer listed – not that that’s a perfect source, but they do take some effort to verify.
The key, of course, is in the little term you slipped in: verifiable. To a genealogist, it’s not enough to simply find someone reputed to be an ancestor, or who was in the right place at the right time to be one – you have to show the link for each generation. And it’s possible to do this with some descendants of Kung Fu Tse, including going what I think is six generations back from him.
There are all manner of less-than-provable ancestries that are probably right, but not proven. One of my great-great-grandmothers, I know her maiden name was Hart, and that she and her husband were supposedly natives of a small town upstream from Albany around 1800. And there is a large family named Hart from that town at that time – but no proof one way or the other that my g-g-grandmother was a daughter of that family.
Likewise a lot of people tie back to Charlemagne, and can document it, or into the House of Wessex (the one you mentioned, I think)-- but they run into a dead end a few generations before that.
If you feel very generous, you can accept legendary lineages, but the serious genealogists will scoff at you. Take a look at O’Neill in Burke’s Peerage – the line goes back through every Irish king who ever gifted a Seanachie to brag him up, winding up, IIRC, with Milesius. (“All together now: Cite?” ;))