There's so many CLAMP titles. I wish I could take a month off from life and read and watch everything. Though I don't think that would be enough time.
My favorite is Magic Knight Rayearth. I love the RPG feel to it and adore the characters.
Second favorite is Card Captor Sakura. Sakura is awesome and I like her relationship with Tomoyo. I like the development the series has even though the episodes are monster(card)-of-the-day like.
CLAMP School Detectives is one of the most adorable shows ever. If you like Ouran HSHC, you have to check out this show.
Snow Goddess Tales is amazing and beautiful. It's a very quick read, only one volume, so check that out if you haven't yet.
I recently started watching and reading Kobato and I enjoy it so far. I can't wait to see where the story goes.