I never really know what to say in these sorts of threads, so I'll just start off by listing the royal houses which hold my interest.
House of Windsor, 20th and 21st century.
House of Bonaparte
House of Palaiologos (and any modern houses which can claim Byzantine descent)
Antiquity in general (i.e., the primeval origins of houses and any connection with late antique families)
The house which I find most interesting--the Julii Caesares--is long dead, alas.
Moribus antiquis res stat Romana virisque