Pretty much every piece from the Season One denouement, Catelyn's ultimatum, Robb's crowning, Daenerys smothering her comatose husband, Jon's honour, The Night's Watch heading out in force to the Land of Forever Winter and the burning of Mirri Maz Dur on Drogo's funeral pyre.
The Finale of Season One (carrying into the end credits), featuring Dany and the return of dragons to the world. The strains of the opening theme for the show at different points just makes it more awesome.
How do you end the first episode of Season Two? With this.
"Pay the Iron Price." Should you ever hear this music start playing, you can be certain that whatever character is the focus of the scene at the time has just made the absolute worst mistake of his life.
The Lannisters' anthem, The Rains of Castamere, by the National. It's heard throughout the second season from Tyrion whistling it to Lannister soldiers singing it in bars 'til it's finally heard in all its glory in the closing credits of Blackwater.
Made infinitely more awesome when you realise that it's a Dark Reprise of Joffrey's "The Throne Is Mine", the Dark Reprise of "Robert Baratheon's theme". Which means that Littlefinger just somehow made a villain song even more villainous!
The version that's played during Jaime's rescue of Brienne is called "A Lannister Always Pays His Debts". Who'd think a song about the Lannisters butchering an entire family could actually be made to sound so oddly heroic?
Ramin Djawadi's score returns in force for the season's first episode, "Two Swords". In the opening the audience is treated to a melancholic rendition of the Stark leitmotif that gradually shifts into a strong orchestral variation of "The Rains of Castamere" as House Stark's ancestral greatsword 'Ice' is melted down and reforged into two Lannister swords by Tywin, which then neatly transitions into the series' Main Theme and the opening title sequence.
From the credits of "The Lion and the Rose", Sigur Rós' eerie cover of The Rains of Castamere. In comparison to the proud, victorious cover by The National, this version would be more apt for a funeral—perfectly symbolizing the Lannisters' new fate. Rome may not have fallen in a day, but the Lannisters did in a wedding. Even more awesome? The beginning sounds chillingly like wolves howling.
HBO is great at finding true alternative pop music gems for the season trailers: