My absolute favorite manga of all time is Petshop of Horrors. I don't know how it went in Japan but in the US, all we got was 4 to 8 episodes which I'm pretty sure didn't even make it to DVD. Sad too because I really loved that series.
A close second though is Fist of the North Star and even though it's dated and, cuts down on the brutality of the manga, it's still pretty good...actually it deviates from the manga somewhat so as to make Shin into a season one arch-villain. Also, like many fighting-based anime, the first series is a couple hundred episodes long. Unlike DBZ though, every episode introduces a new villain for that day who dies by the end of the episode (for the most part: Jackle, Shin, Sauzer and a few others last a couple of episodes)