I've seen a few episodes of last airbender but didn't really get into it.
The most mainline anime I've watched through is probably Naruto. That had some good parts but also had parts that dragged.
Bleach has a few hundred episodes and somehow manages to be mostly good. IMO most of the anime that continue on for over 100 episodes end up dragging in parts. But Bleach managed to maintain good production quality for most of its episodes.
I watched about 300 or 400 episodes of One Piece. I wish I hadn't.
not an anime fan, I still prefer Bugs and Daffy and Foghorn Leghorn. One exception -- The Last Airbender. I wore my yip yip Appa tee shirt the other day.