Weeds (First two seasons are world class, but took a major dive in quality)
Arrested Development (Not a single weak moment in the entire series)
It's Always Sunny in Philadelphia (I've heard it described as Seinfeld on crack)
Weeds was good for the first THREE seasons, THEN it took the huge nose dive into mediocrity.
Arrested Development is perfection.
It's Always Sunny's first 3 seasons kick ass, I think season 4 or 5 was the weakest link but then it picked right back up again after that.
Also giving Firefly another shout out since I just finished it and thought it was damn good.