Princess Tutu is oddly enough, one of my current favorite series. Partially it's because, like Fruits Basket, once one gets past the cute exterior, the story itself is deadly serious. Partially it's because of the realization "Oh My god, this is a HORROR story!"

Ffnally, I love it because it's a series that really depends on a knowledge of opera, as the episodes are heavily based on various operas. Which is why I got to the point of saying "Oh no! Not THAT theme!". I grew up on opera, so I appreciate the references.