What do Prince Charles (Prince of Wales), Harald V (current king of Norway), Nicholas II (last tsar of Russia), Constantine II (former king of Greece), and every king of Denmark from 1448-1972 have in common? They are all male-line descendants of the House of Oldenburg.
The Oldenburg dynasty can be traced to an 11th century count living in Lower Saxony. It rose to prominence in 1448 when Christian I became king of Denmark. A cadet branch (i.e. a junior male-line) of the family known as the House of Holstein-Gottorp also went on to rule both Sweden (1751-1818) and Russia (1762-1917).
The main line of the House of Oldenburg died out in 1863 and at that point, a cadet branch known as the House of Schleswig-Holstein-Sonderburg-Glücksburg (or just Glücksburg for short) became the senior branch of the family. But not only did the Glücksburgs inherit the Danish throne, they also went on to rule Greece (1863-1973) and Norway (1905 to present).
Perhaps most interesting though is the fact that a male-line grandson of George I of Greece (known today as Prince Philip or the Duke of Edinburgh) married the princess who would go on to become Queen Elizabeth II of Great Britain. This means that the next three individuals in line to the throne of Great Britain -- Prince Charles (the Prince of Wales), Prince William (the Duke of Cambridge), and the young Prince George of Cambridge -- are all male-line descendants of the House of Glücksburg, and hence also of the House of Oldenburg.
So does this mean that the future reigning house of Great Britain will be called the House of Glücksburg? Probably not. For one thing, Prince Philip considers himself to be a member of the House of Mountbatten (named after his mother's family). And second, Queen Elizabeth has stated that she wants the name Windsor to be used for the British royal family for perpetuity, regardless of how the throne gets passed. Of course, the next monarch could change this but that seems unlikely.
So forget about the Habsburgs or Capets. Perhaps it's the Oldenburgs who will eventually be remembered as Europe's greatest royal dynasty.