30 Rock
Louie
Always Sunny in Philadelphia
Arrested Development

It's hard to organize them. I rewatch all four all the time.

True, but they ended up not using him much, and it was relatively one small thing over 6 seasons - hardly show-killing. The great stuff definitely more than made up for the few things that weren't so great, IMO.

My best/favorites would be:

Game of Thrones
LOST
Friday Night Lights
Veronica Mars (Amazing first season. Second season was pretty good.)
Curb Your Enthusiasm
The O.C. (I think the first season is the best ever of the genre. It has everything and every holiday episode they did that first season was a classic. Unfortunately, It was a long 27 episode season and I think the writers were wore out after it and the writing didn't pick back up again until the fourth and final season.)
Downtown Abbey
The Amazing Race
Survivor
Battlestar Galactica

It was divine providence we got three magical seasons of Julia Shumway's adventures.

My answer is Lost, but I am one of those people that have blotted out the last season from memory. Lost had five brilliant seasons, which is good enough for me.

I would have said Game of Thrones but the past two seasons have dragged its lofty rating down.