Do shows that didn't get accepted for renewal count?
For me Rome and Stargate Atlantis were big ones.
Rome was an awesome show with great actors, and some say that Game of Thrones replaced it as the go to show. In many ways, it is basically the same show. I never understood with all the bad shows on HBO they couldn't find a spot for Rome.
Stargate Atlantis demonstrated the end of real Sci Fi on television as far as I am concerned. But to be fair Sci fi had been struggling with a lot of filler episodes for a long time. I just felt Stargate Atlantis had so much potential.