Created an account for this.
My first choice would be Megas XLR. That show was amazing, but only lasted for 2 seasons. Every single episode was awesome and hillarious. Then at the end of the second season, it just dissapeared, with soooo much potential left.
My second choice would be Teen Titans. It did NOT have a satisfying conclusion. They could have chosen to end the series after that big battle, when the brain dude was defeated, and everything would have been fine. Instead, they decided to have one more episode after that, where Terra was rivived. All that episode did was create even more questions without ever answering any of them. The series then ended after that.
I would have picked Ed, Edd, and Eddy, but that show had a satisfactory ending with the movie.