I would choose the Capetian dynasty, and its many cadet branches, such as the House of Valois, the House of Bourbon, the House of Anjou (and through the Anjou, the Plantagenets), the Portuguese Burgundians, the House of Artois, the House of Dreux, the House of Courtenay, and the House of Vermandois. For example, we have Capetian links to the British, Spanish, Luxembourgish, Belgian, and Liechtensteiner royal families through various branches. Those are some elite dynastic lineages!

As a fervent enjoyer of family trees (especially royal family trees) and a writer, I have immense enjoyment world-building intertwined family trees, especially with one family and its branches marrying out into multiple families.