I love Young Justice and it will clearly win the poll, but I have to give my vote to WATX.
The difficulty in writing that show was pretty extreme because of the two different settings (present and future), so you have the X-Men of the present trying to correct a future gone wrong with bits and pieces of future knowledge. It was a very nice mix of X-Men not an overly powerful group (as far as X-Men go) which meant they had to work as a team. The best part was that the characters were true to themselves.
I would have loved to see a 26 season long episode story line dealing with the AOA. You have to wonder what would cause Scott Summers to ally himself with Mr. Sinister.