Dead Like Me
Honorable mentions - To be fair most of these probably deserved to be canceled (except Jericho), but I would have enjoyed seeing where they went.
The Adventures of Brisco County Jr.
Terminator: The Sarah Connor Chronicles
Andy Richter Controls the Universe
Pirates of Dark Water
Misfits of Science
Hardball (not the Chris Matthews one)
I'm tempted to say The Wire, because it's such an amazing show and I suspect they could've focused on one, maybe two, more institutions and then quit (I believe Simon planned to do the Church and then the TV media). However, the ending felt clean anyway, so I'm satisfied either way.