With the movie coming out this week, I took to completing Books 2 and 3 in one glorious session this past weekend. The show is a fabulous watch, definitely one of the most enjoyable serials I've had the pleasure to experience in a long, long time.
What follows is the long-winded prattling of a newborn fangirl. I'm not going to bother whiting it all out, so be warned that there are plenty of *BIG SPOILERS* ahead:
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It's a tough pick, but I'm gonna say Sokka is my number one favorite character. Right from his first scenes in the very first episode, I fell in love with the meat-and-sarcasm-guy's comedic hijinks. Foo Foo Cudley Poops made me laugh so hard. Sokka's art drawings rock, and I'm glad he found his true worth in battle through his swordsmanship with Sifu Piandao.
Toph is a very close second. I'm so happy the creators opted to make her into the sarcastic, kick-ass 12 year old that she is. Her self-depreciative blind jokes are always good for a chuckle. "I AM MELON LORD!! MWAHAHA!"
Bumi is my favorite secondary character. I love the idea that there was someone who knew Aang a hundred years ago that can still be around today. "The King of Omashu" is one of my all-time favorite episodes; it gave a first taste at how intricate these Element battles can be. The Bumi-isms definitely stuck with me: "First of all, it's pretty fun messing with people." "Lettuce leaf?" "Who are you calling old? Okay, I'm old."
The voice work is sublime. Jessie Flower really brought that sense of precociousness to Toph, but the likes of Mark Hamill as Lord Ozai and Jason Isaacs as Commander Zhao definitely gave the series a killer edge. John DiMaggio, Phil Lamarr, Tara Strong, Robin Atkin Downes, Cam Clarke, James Hong, Paul Eiding, and Quinton Flynn were also terrific guest additions. Grey Delisle ended up being one of my favorite voice actresses, her Azula reminding me of Super SFIV's Juri: gleefully sadistic, pure villainy. I'll need to re-watch the series to study the differences between the late Mako Iwamatsu and his replacement for Book 3.
On that note, Iroh is another awesome character. Him playing the surrogate father figure to Zuko was handled well, the long awaited reconciliation scene between the two after the events of Book 2 a wonderful turning point in their relationship. One wishes he were utilized more in Book 3 (could his lacking presence be because of Mako's recent death?), but his fall from Fire Nation general to rising tea shop entrepreneur made for a great story arc. The White Dragon / White Jade Bush: delectable tea or deadly poison?
"Appa's Lost Days" was a great way to retell what exactly happened to the lovable beast. (As a side note, "The Desert" episode had to be one of the most emotionally weighty episodes in the series. It's one of the few times where Aang is in an uncomfortably perpetual state of angst.) It's almost heartbreaking to see the big guy fall into such a feral state. It felt entirely real that any animal that undergoes such abuse by human hands would become as untrusting as he had become. Having Zuko save him was a great setup for his eventual alliance later on; that even Appa has a say as to whether or not he wants Zuko to join was a nice touch.
Favorite element? Each episode manages to find inventive, new ways to use them, so it's hard for me to narrow it down to one. The "hidden" second level of bending was a rad idea. So, Earth has Metal, Fire has Lightening, Water has Blood..... where does that leave Air? Does it even have one?
Bloodbending! Man, was "The Puppetmaster" a creepy episode. It's great that they've finally shown a dark side to Waterbenders. Hama was a great villain, and having Katara learn the skill against her will was a delicious way to end the episode. Sugar Queen no more.
I loved how the gang underwent cosmetic changes as the series progresses. Sokka and Katara go from their Water Tribe winter clothing to warmer weather-based gear by Book 2. Zuko and Aang grow their hair out, and Book 3 has all of them going incognito in Fire Nation outfits. (The Fire Nation outfits are far and away the coolest, by the way.)
The Momo Yojimbo/Appa Samurai duel was batshit, out-of-left-field insane. Loved it.
The storytelling approach in "Tales of Ba Sing Se" was another favorite of mine. That all the short stories came together toward the overall plot, rather than being just a bunch of lighthearted 'what if' takes, was confidently executed and wholly enjoyable. Yet another favorite was "The Ember Island Players." Now THIS is a clip-style show done right. Woman Aang! Man Toph! Jet's ambiguous death, Zuko's blatantly growing hair, the deliberate skipping over of The Great Divide chapter..... I love that the creators weren't afraid to poke fun at the show and rein in truth at several turns. The reactions from our main characters were truly priceless. 
Zuko's story did indeed kill. Honestly, the way he was carrying on, I never thought in a million years that he'd end up being Aang's new Firebending master. Though I'm delighted that he turned to the side of good by the end, the initial approach in introducing him into the gang was something of a strange and awkward watch. Maybe it's because his personality does such a complete 180; at best, one could say that he's just so bad at being good.
I kind of wished they had him join the group earlier on in Book 3, but the few remaining episodes where he's featured alongside Team Avatar made for terrific chemistry. I just love his straight-man presence in the group, his story gave him shots at redemption with the core members. (Sokka in The Boiling Rock, and Katara in The Southern Raiders.) The final Agni Kai with he and Azula was a thrilling confrontation.
I will say that I hated how each episode from Book 2 onward had to have a "Previously on Avatar...." recap at the beginning of each one. With so much going on story-wise, I knew they were there to get viewers up to speed every week -- but it definitely managed to spoil some of the surprises in store. For instance, they broadcast the return of Suki with a big, loud bullhorn. From that point after, I decided to just skip through all the recaps.
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Bring on the movie. Spoken now as a rabid fan, I hold a far more pessimistic view that Shyamalan can screw this up pretty bad. We shall see.