Without exaggeration, this is one of the greatest television series of all time. Other writers and directors should take notes. A few major high points:

Every character is rich, many undergoing incredible, complex character arcs, while others simply briefly add to the world.

The pacing is exquisite. Occasional filler episodes break up the steady march of the overarching plot, but they are some of the best episodes in the series ("Tales of Ba Sing Se" and "Ember Island Players" come to mind).

The world building is unparalleled. Inspired by real world cultures and mythologies, yet with enough original content to be completely unique, you constantly want to see and know more of it.

The writing as a whole is pristine. Overall it gives a strong impression that the writers always knew exactly where they were going. Old plot points resurface in clever ways throughout the series, twists are surprising but feel right, and I can't think of a single thread left untied at the end.

Basically, nothing could be improved about this show, and it's such a perfect story package that I hardly even wish there was more of it. (I'll just rewatch it a 5th time.)