Hmmm...
For me, it's Sakuya hands down. Her headpiece is just what makes a maid.
Others I like are Yukari's, that ribbon on it... Damn...
Remilia's mop hat, it's big and frilly, unlike Flan's which is small and poofy.
Oh and Tenshi's! Those peaches with a combination of black gives of awesomeness, and I want to nom on her yummy looking blue hair, too.