Avatar: The Last Airbender - the best cartoon/anime I have ever seen. There are several things that make it great and I will enumerate them one by one.
First, the originality of the series. The thought of a world wherein people that can control the four fundamental elements called benders exist. That's a first in history.
Second is the action. The fight scenes weren't just blurred unlike several animes like Naruto, no offense Naruto fans. When benders fight, you'll see every strike, every form, every stance and every movement of the characters. What's better is that they're all correct since the writers did their homework and hired a real martial arts consultant - Sifu Kisu.
Third, the anonymity. You didn't really know who or what the characters really were. Aang never revealed his history as to why he was trapped in the iceberg until episode 12. Same as Zuko as to why he got that scar and why he is really hunting the avatar. Iroh wasn't just "that wise old man" in a series, he had a complicated past. Best of all, Firelord Ozai's face wasn't shown until season 3. His face was always covered in shadows and that's what made him more frightening and evil.
Fourth, the antagonists. For a kid's show, the antagonists were pretty deep and well thought out. Some of them were really bad to the bone. A banished prince unsure of his destiny, fighting within himself and his confusion of his real personality. A twisted princess born with talents and incredible bending feats but is jelous of his sibling and was more than just 'sibling rivalry.' And lastly, the terrible Firelords. So evil that one firelord dueled his own son! So evil that he banished his own wife! So evil that one ordered his son to kill his grandson! So evil that one left his best friend to die! So evil that one even lead a genocide!
|"You've beaten me at my own game."|
|— Long Feng|
|"Don't flatter yourself, you were never even a player."|
|— Azula in response to Long Feng|
Fifth, the humor. It's a hilarious show, need I say more? It's the closest Nick has ever gotten to showing drugs on a show!
Sixth is the music. The series' soundtrack gave you emotions while watching the show since it's created and placed so well. Whether it's a fast paced theme that made you jump out and punch like "Agni Kai," the stealthy tune of "The Blue Spirit Theme," or the creepy music of "The Dai Li."
Lastly, and probably the best part, is the story. What's good about the Avatar's story is that it's well planned out! Before every episode could even air, the writers already made an ending for the series. Every section of the series was plotted out and made sure continuity played a role. This is what made it successful, it ended. Unlike Naruto, although it was successful, it just went downhill when they started to make stories as the manga goes. In my opinion, this was the element that made Avatar so successful.
Even though the series seemed perfect, it's not. There's still the fact that it's a show that were open to kids. Meaning it minimizes violence and refrains from showing blood and gore.
Despite of that, it lives up to its name as one of the best animated series ever made.