I like all of these equally: Vampire Knight, Fruits Basket, Fullmetal Alchemist, and Princess Tutu. :3
I was going to throw Elfen Lied in there but.. it's good but I didn't actually like it as much as the above ones. The above ones I am willing and have watched it more than once and Elfen Lied, even though I have the whole DVD set I don't like seeing it again xD