Arrested Development
The argument for "Best Sitcom Ever" is nowadays often made on behalf of Seinfeld. I always appreciated that show (and continue to watch it even today, 25 years after it went off the air) but I'm not sure it makes my top three.
The show firmly entrenched in my top spot is Arrested Development, as carefully orchestrated a display of familial folly as any ever aired. It's a joy to watch so many talented actors romp around with such terrific writing.
Oddly enough, I think the show's commercial failure -- it was a ratings dog and ran for only three seasons, the last of which was just 13 episodes long -- contributes to its greatness. Many shows start out strong but simply lose steam, something Arrested Development never had the opportunity to do. (Recall the disappointment with which the aforementioned Seinfeld left the air.) And the series ended on a great note, capping its run with an episode that offered just the right amount of closure without totally slamming the door on future tales.
As proof, here's that "great note," the final episode of Arrested Development. Never seen an episode before? Watch this one anyway, and I'm sure you'll want to go back to the beginning to get the full story.