Dexter and Six Feet Under, had they only lasted one season, would have left me remembering them as great shows.
Stargate season 7 or 8 would have made great finales.
Babylon 5 season 4.
Heroes season 1 is also a super obvious one. Or 24 season 1, 2 or 4.
Also Flashforward, though IMO it deserved better ratings than it got, went out on a very satisfying note.