Cool idea for a show.
My nominations:
Firefly seconded. The cancellation I mourn most.
The Tick. Both versions (the animated version was better, but it did get 2.5 seasons. The live-action version got snapped off at the roots.). Apparently the first season of the animated version is coming out on DVD, but missing one episode for some sort of legal reason.
Carnivale
Witchblade
Crusade
Dead Like Me
Jeremiah
Frank's Place