I've seen only a few good shows listed, I'll reiterate them along with all the shows you monkeys missed.
Firefly; couldn't have gone bad even after 10 seasons
Futurama; had the potential to last longer than the Simpsons has (someone should create a similar thread for shows that should have long ago been ended, like Law & Order)
Enterprise; knew it was dying, so it whipped out the good stuff at the last moment. had little potential because of linearity constraints which they totally fucked in the Star Trek remake anyway.
Farscape; last season was balls in the face, but the creators themselves said they had 9 seasons planned.
Dead Like Me; could have gone another couple seasons, but there was never much room for expansion beyond the realm of a sitcom.
Sliders; literally limitless potential, but with casting issues axing every compelling character, funding issues leaving the scifi thriller in the show in the toolshed production making every episode more and more like old horror flicks, the show contracted leprosy and fell apart as inevitable as the tides.
Aquaman; lots of potential, but picked the wrong damn superhero to make a trendy show out of on a beach. Now Wonder Woman and her trysts on the island full of Xena-like babes and her adventures into the forbidden realms of man would have made a VERY compelling show.
Angel; as soon as Buffy The Vampire Slayer ended its 7th season, Angel barely limped on one last, terrible, plot-slaughtering season. If Greenwalt and Whedon had kept things safe and familiar that 5th season they could have done their Wolfram & Hart exec team season later on, but they changed the show too much in that season and lost their core, girl power fanbase by killing off Cordelia and Fred alike.
The Invisible Man; Still not sure why this show ended, other than having a 70's cop show about a sassy bald white guy and his wanna-be ghetto invisible white partner with a dead brother that lives inside his brain story-line. But I loved it, miss it, wish they hadn't axed it, but whatever, I felt like listing it anyway.
Now there are other shows I could have listed as well like Stargate: Atlantis, Lois and Clark: The New Adventures of Superman, Hercules, Xena, all shows that could have gone much longer but their stagnating story-lines demanded execution anyway.
If you feel the need to dispute me, obviously you read to the last page of the thread which NOBODY EVER FUCKING DOES so I'll end my thread with the following close: why did I write all this when nobody is going to read it anyway?