The absolute first CLAMP anime I ever saw, was
Miyuki-chan in Wonderland because I thought the drawings were beautiful. This was waay back in '96. But, that's not their best work. I'd have to say that
Cardcaptor Sakura is their best work, because the characters are great, the story is nice, and the atmosphere of the anime itself is postive and a nice feeling. I've seen
Magic Knight Rayearth which I didn't like as much and
Tokyo Babylon which I love. Then I saw X the Movie and hated it.

There are probably more that I missed of their animes, but as for their mangas, I've flipped through volume 2 of Clover and didn't like it. There's also this manga with a girl holding a bear and I thought the drawings were interesting and different, so I got those. But, CLAMP overall.....is a very depressing and realistic bunch of women. They're not all sugary and happy and people don't end up together when you wish they could be (SPOILER- Sakura and Yukino in
Cardcaptor Sakura). So basically, CLAMP's works are just plain beautiful to look at, but I don't really like their stories that much.