Daenerys with her brothers Viserys and Rhaegar.
Now comes mine imo, I do not pretend it to be absolute truth. Always thought that valyrians and their descendants were considered beautiful not because all of them were like Hollywood stars, kind of Tolkien's elves, or overused frank fineries with make-up. But because they incredibly stand out of the crowd (even noble). Albinos (only eyes not red but violet) with marble skin, chiseled features, graceful, refined in dark luxurious clothing. In addition with ambitions, temperament and dragoncontrol thing. When someone along those lines appears among dothraki or ghiscarians it is natural that they would proclaim Deny "the most Beautiful". Taking into the account sweet atmosphere of Middle Ages: dirt, diseases, dust, death all around, etc.