Card Captor Sakura
One of CLAMP's masterpieces, Cardcaptor Sakura takes a slightly different twist on the 'magical girl' anime genre. Instead of focusing on the adventures of teenage girls imbued with magical powers, Cardcaptor Sakura focuses more on the evolution of a ten year-old girl, Sakura, and her relationship with the world and those around her. While each episode presents itself with a simple premise of capturing the magical incarnations of the cards Sakura mistakingly released, it also takes great pains to define Sakura's character through the bonds that she makes.
One of the more interesting aspects for me is how Cardcaptor Sakura is, plotwise, the predecessor to other CLAMP works. Cardcaptor Sakura introduces the Buddhist concept that the those around you will be reincarnated to continue their journey with you. Not only is this evident in Cardcaptor Sakura's character of Clow Reed, but also in Watanuki/Yuko's relationship in XXXholic.
As I continue collecting Carcaptor Sakura artwork, I find myself gravitating more to certain episodes, rather than certain characters.