Fic Title: Stormbenders
Why This Must Be Read: One of the best fics in the fandom. : S3 AU from FBM. Deep in the Fire Nation jungle, the Gaang discovers a rogue tribe of exiled water ninja who send Zuko and Katara on a mission to recover the Fire Nation's secret battle plans. Long, plotty, with great characterization and a wonderful look at the growing dynamic between Zuko and Katara.