I have to say that Tsubasa RESERVoir CHRoNiCLE is my favorite.
It features Sakura and Syaoran from Cardcaptor Sakura, only older, as well as a few original characters, such as Kurogane and Fai D. Flowright. CLAMP also includes several characters from other works that they have done, and it crosses over with XxX Holic.
In my opinion, this series is absolutely amazing. The plotline, the artwork, the character depth is completely amazing. You should all read it.
Lots of pictures, I know. Couldn't help myself. They're just amazing to me. =3