Beck, Gantz, Kaze no Stigma and Dragonballz.
Dragonballz because it was such a simple story that was easy to follow and easy to enjoy(as long as you hadn't gotten into the seinen genre before watching).
Beck because the manga is still going and yet theres no more anime, and due to the lack of manga OST's its hard to appreciate Beck's music.
Gantz because the ending was so retarded. It should have taken a long break after the filler and then came back with a new arc(that was in the manga).
Kaze no stigma because its such a good show, and reading it wouldn't be the same as watching the relationships evolve.