Firefly - Just such a great cast and writing.
Arrested Development - Fucking hilarious, the satire is incredible (S1-3)
Avatar The Last Airbender - This is part of my childhood and still holds up today.
Legend of Korra - Seasons 1 and 2 were sub-par, season 3 on the other hand has been up to par with ATLA.
Game of Thrones - So D&D are kind of diminishing the ASOIAF series in my opinion. Subplots and changes that are completely nonsensical and an obsession with sexualised violence that isn't even present in Martin's novels have pissed me off heaps, but there's no denying that the universe is just too damn rich and addictive to ignore.