Avatar: The Last Airbender. No contest. It is a fantastic show that delivers in practically every area. The characters have fleshed out personalities with solid development, Zuko's storyline in particular is excellent, the storyline is paced quite well, it does a great job with its worldbuilding, the action is great, the humor is a good balance to the action and serious moments and it had a really satisfying conclusion. While I can see why people would find it overrated, this is one of the few series that I think deserves every praise it gets. It was terrific and easily one of the best animated series in recent years.
I remember watching Aladdin: The Animated Series back when Disney Channel aired it and I remember having a lot of fun with it. It had some good adventures and it was engaging, but I don't know if I'd still feel that way if I watched the series now. Even if I did, it would have had to be extremely good for me to vote for it. As it is, Avatar: The Last Airbender gets my vote.