Unsurprisingly, one of the best geneologies in terms of records, probably THE best (but I'm not 100% on that, so if anyone knows a better one do say) is Elizabeth II. Elizabeth, as I'm sure you're aware, traces her lineage back to William the Conquerer. However, she in fact can also trace her lineage back to the House of Wessex. After the Danes were driven out of England, Edward the Exile, son of King Edmund Ironside, returned to England from Sweden. King Edward the Confessor, who was childless, named him his heir, as he was the only other man left of the House of Wessex. However, 2 days after arriving back in England, he died (a bit suspiciously). After the Battle of Hastings, his son, Edgar the Aethling was briefly declared King, but his supporters abandoned him and he surrendered. Later, he moved to Scotland, where his sister Margaret married the Scottish King Malcolm. Their daughter Matilda married the King of England, Henry I.

So, Queen Elizabeth can trace her lineage back to Matilda of the House of Wessex, and thus back to King Edmund Ironside, and thus back through the House of Wessex. The founder of the House of Wessex was (probably) a Saxon named Cerdic. He probably arrived in Britain in 495, and became King in 519AD. However, some have suggested he may have actually been a British low-ranking ruler who merely rose to Kingship, as Cerdic is a Brittanic name, not a Germanic one. The dates are probably not 100% reliable either, as they come from the 9th century Anglo-Saxon chronicle, but he was probably the real founder of the dynasty nevertheless. As to Cerdic's ancestry... well he is supposedly descended from Wodan (yes, Queen Elizabeth can trace her lineage back to a god) on one side and Abraham on another, so it's assumed that his ancestry is entirely made up.

The oldest extant baronial title in Britain is that of the Baron de Ros. The current holder is the 28th Baron de Ros. The writ creating it was issued in 1264. There are some older extant titles of dubious claim (that are likely recreations of more ancient titles that died out), and also a couple of baronies that have been absorbed as subsidiary titles by later creations (Baron Kerry and Lixnaw, dating to 1181, for example, was absorbed by and is now a subsidiary title of the Marquessate of Landsdowne). The first Baron de Ros was the grandson of Everard de Ros (d.1227), Baron Helmsley, who was one of the 25 barons who guaranteed observance of the Magna Carta by King John. Everard de Ros was a descendant of Peter de Ros, who was recorded as Lord of the Manor of Helmsley at the time of King Henry I (c. 1100-1135), so just after the Norman Conquest (Henry I was a son of William the Conqueror). Burke's Peerage and Cokayne's The Complete Peerage both list complete geneologies on the history of this title and family going back to just after the conquest. So it is pretty well recorded and verified by land ownership records, etc. Other families holding ancient titles like this almost certainly have similar records. So there are certainly some families that can trace lineages back that far, on paper, anyway.

Edit: spelling