Yu Yu Hakusho. It has an interesting plot with a successful blend of science fiction, drama, action, comedy, philosophy, and romance. Practically all of its main characters are not only likable, but also have complex, well developed personalities and undergo fantastic growth over the course of the series. The fights within the series are also exciting and will keep one on the edge of his or her seat from start to finish. Additionally, both its animation and music are beautiful.
Neon Genesis Evangelion comes a close second. It is a psychological series with a cast of very flawed and damaged yet fascinating people. While I can certainly understand why one would find the series' content disturbing and/or be annoyed with its cast, I personally consider it to be a deep, thought provoking work of art. Both its music (ah, Cruel Angel's Thesis ) and animation are also top notch.