Even though it isn't running anymore, mine would have to be Avatar: The Last Airbender. It is just...a perfect প্রদর্শনী (the sequel is NOT inculded. I was majorly disappointed with Legend of Korra).
The runners up are both no longer running either: Danny Phantom and Jimmy Neutron.
They are all cartoons, so what? In mt opinion, Nick hit it's prime with the early to mid 2000s cartoons.
