Everything from the Grand Finale, particularly the music playing during Zuko and Azula's Agni Kai, appropriately titled "The Last Agni Kai". It is dramatic and intense, and its mournful tone underscores just how tragic the situation really is.
The Blue Spirit. When you hear this, you know whatever happens next is going to be BAD. ASS.
Season 3 Trailer Music, and the remix used in the sequel series trailer: Pure Awesomeness. It turns out that the "remix" was simply a video speed error that sped up the original song, but the "wrong" version is still pretty cool.
In subsequent airings of "Sozin's Comet", the four parts were broken up, with a "Previously on Avatar" recap for each. The music playing during the recap for "Part 4: Avatar Aang", is pure, distilled awesome, and you just know that whatever happens from there on out is going to be spectacularly EPIC. There's a short, high-quality version and a full-length, low-quality version. Prepare to feel the AWESOME.
"Leaves from the Vine," for all its sadnessnote both in-universe (where Iroh sings it before an impromptu shrine to his late son, Lu Ten) and out of it (as Mako-san recorded it in his last session before he died), is also a very soothing song, with its gentle strings and Iroh's soft, understated vocals.
The awesome battle drums in "Invading the Palace", when the Gaang make their way to the Earth King in the eponymous episode.
"Tsungi Horn," which is considered Zuko's primary motif (playing during scenes where he's at war with himself over his motivations), is just downright haunting.
It's a damn shame that "Scarf Dance" (which was written for a scene in "The Headband" where Sokka was supposed to dance by himself) never made it into the show proper — it's stirring and zesty, and will make you want to dance along.
"Agni Kai" — the track that accompanies Zuko's duel with Zhao — is sure to set your heart pounding.
"Iroh's Speech", the soundtrack for when Iroh rallies Team Avatar (minus Aang) to go forth and face their destinies in the final battle on the day of Sozin's Comet — and to have faith that Aang will defeat Ozai when it all comes to an end.