This is said to be QE II's favorite:
It's a little to plain and pointy for my taste. Speaking of pointy, check this out:
I'm partial to sapphires so this one goes to the top of my list:
It's owned by one of my favorite royals, Queen Maxima of the Netherlands.
I can't decide if this is my favorite:
If you have diadem dreams, check here for a 101 and here for the ultimate collection!