Fire Lizard In A Bottle's review
Although the main relationship is Zutara, the main draw to this fic is the wonderful world-building. The author has done their research and managed to create a realistic world much like our own with bending. The subject of slavery and the different cultural clashes between Fire Nation and Water Tribe is handled in a tactful and interesting way. Absolutely recommended as one of the top Avatar fics out there.
The slave isn't who you might initially think. The setting is based on an Avatar world with no Avatar but all the bending intact. Each of the four nations has its own language (sometimes more than one!) and culture, and the story takes full advantage of it by following the trials and tribulations of the captured slave forced into a completely alien community. There's a little Dances with Wolves flavor here, but it's not as one-dimensional. And if world-building isn't your thing, there's plenty of cute Zutara moments.