The Simpsons - Consistently great writing, acting and animation (and by that I mean almost every single episode from seasons 2-9 was a classic) for about 8 years, which is unheard of in television. Shows become profitable around the fourth season, when they can be sold into syndication, and the quality usually drops off after that because everyone gets rich and doesn't care any more. The show is bad now, but if they want to drive it into the ground and make more money, that is their right for creating something so great at its peak.
Twin Peaks - It went off the rails after the first season, but that first season was genius. So quirky, great characters, great premise with a terrific MacGuffin. The girl's murder is totally unimportant; it just served as the flash point to draw you ever deeper into an increasingly bizarre world.
Seinfeld - Seinfeld declined halfway through its run (around season 5 or 6 - see above), and really started to suck when Larry David left the show. But during its early days it was weird and funny and original, very unlike the other sitcoms that came before.
Arrested Development - Perfectly cast. Perfectly acted. Shame America prefers crap like Two And A Half Men and The Big Bang Theory.
The Office (US) - I love this show. It took a while to get going and get out from under the shadow of the UK version, but it stood on its own legs for a few seasons and really delivered. Now its in decline, and probably won't last long after Steve Carrell leaves.