Aren't "coolest looking" and "favorite design" pretty similar concepts? I think coolness is probably just as subjective as favorite design. There are no rules of cool.
That being said, I think Xatu is a pretty design-y pokemon. I really like its color palette and it's geometry. It's reminiscent of Native American, Mayan, or Aztec art, which makes it unique in my mind, as not a ton of other pokemon are as influenced by culture.
Also, it's interesting that it is a psychic-type, implying the link between ancient peoples, intuition, and spirituality.