While not specifically atheist, the "fortuneteller" episode reveals my favorite character, Sokka, as one of the best skeptics I've seen in a children's show since Chirp from "Peep and the Big Wide World."
It also has a great example of how exasperating and frustrating "believers" can be to such a skeptic.
I totally sympathize with Sokka's plight! Even if you don't know anything about the show, I still think you'll get a kick out of watching it.
Besides... it's free and entertaining.