I don't know what it is but when I first saw Eureka seven I went crazy about it. Which at the time meant a lot because I knew at the time it had been a very long while since I had held that much enthusiasm towards an Anime. You could argue that maybe it was the art/animation that wow'd me or the Childish innocence that was very appealing and nostalgic. Idk I just thought it was good, man.
Though of course for least favorite,
Yumeria would be an easy shot.