In my mental compendium of shows that got canceled for no reason, here are my 5 favorite ones. By sheer coincidence, they are also the universal top 5 best ones. No, Firefly is not on this list.
# 5
Constantine
In 2014, NBC had the crazy idea of assembling an elite team of writers to create a show based on a witty Englishman that comically fought demons by fiddling around with horribly powerful arcane relics he has no business fiddling around with. It was an incredible mix of cynical humor and philosophy that managed to get a cast of characters that were far from unmemorable. It was canceled after a single season since NBC stands for something that implies it's run by morons.
# 4
Columbo
Hang on dumbo, Columbo was never canceled. The show was made in 1971, hiring Peter Faulk as a socially awkward detective that was a savant at solving cases. Yes, in 10 seasons, he managed to solve every case in the first 25 minutes of every episode, but at least he was entertaining. The show was wildly successful, even getting a spin-off show about his wife (terrible). They revived the show in 2003, but due to young folks not wanting to watch an old man bother people, the revive never picked up.
# 3
The Borgias
Airing in 2011, The Borgias was about the Borgia family's unethical misadventures after the head of the family, (played by Jeremy Irons) Rodrigo Borgia becomes the Pope in the 16th century. The family was involved in a slew of scandals and murders based off of real history, creating one of the most compelling and emotional historic television series in history. The show was cancelled after 3 seasons for... probably bull crap.
# 2
Stargate Universe
Stargate Universe was the widely anticipated spin-off of Stargate SG1, featuring a cast of scientists and solders that are forced to survive together while stranded in a derelict alien ship billions of light-years out in space. It was one of the most successful mixes of science fiction and drama, but it was canceled since SyFy simply refused to leave it in one time slot in season 2, loosing thousands of viewers due to complete incompetence. It currently is running a page called Save Stargate Universe, and so far, over 100,000 people have been requesting Netflix to revive the series. I have checked the page every day since 2011, waiting for the show to come back. It will.
# 1
Salem
Yea, yea... my favorite show of last year was canceled. I've told you already that it is 100%, without debate, the most well-written television show ever created, but WGN does not know how to properly advertise, so the show eventually just failed. Strange, since the only episode that was criticized by the fans was the final episode, which was forced to rewrite since it was cancelled in the middle of its gawd dayum last season. Salem is a show that made no mistake except being ahead of its time on a dying network and I absolutely expect someone actually qualified to pick this series up and continue it. What a shame.