Byzantine minors, Alan ruling house, de Normandie, de Toulouse, Knytling, Natt och Dag (awesome name), von Habsburg
Vermandois, Bagratuni, Rurikovich, Palaiologos, Dongola, Komnenus,
Really it depends on what area of the map I want to play. I also often look for the oldest families in terms of family tree and choose that way as well.
First game no doubt will be as Georgia, Nubia, or appointing Gagik as duke of armenia and playing as Bagratuni. In an eastern mood. Probably play as a celtic family after that.
What were some of the older families in CK1?
Going backwards through the CK1 character sheets, I find that de Flandres are direct descendants on the male line from Charlemagne's son Charles the Younger and the Vermandois from Carloman. The Capets also trace their ancestry to the ninth century. The House of Barcelona likewise goes back to Count Borrell who IIRC declared independence from West Francia. The Jimenez Kings of Castilla, Leon, Navarra, and Aragon in 1066 are all descended from Jimeno, whose origins date back to the mid-IX century. So in the West, some dynasties are of old origin, often margraves and other rather minor officials in the Carolingian empire who rose up in the power vacuum of the X century.
Everyone's starting dynasty, the de Hautville. Or maybe the Count of Moscow, unite Russia and become the slayer of the Ruriks! Or maybe the Komnenois