Ferelden at War, which plays during the battle at Ostagar. The subdued opening slowly builds tension as the darkspawn come out of the forest until the armies finally charge. The sequence is bookended by the equally awesome The Betrayal.
The Party Camp. It's only a little minute-long snippet, but it's beautiful. Especially considering that the camp is where you will end up having most actual conversations with your party members, which means this is the song that plays during many of the game's more emotional moments.
It's odd how the vocals in that piece always seem to arrive at the right moment during your conversations with party members.
Ruins of Ostagar perfectly conveys the enormity of the place and the grandeur it once had. NOTE: Despite being called "Ruins of Ostagar" on the soundtrack CD, this theme is never used in the Ostagar scenes. It is actually the Orzammar theme, and is used there. The actual Ostagar theme can be heard during the first 40 seconds of the "Betrayal"track on the soundtrack CD.
Whatever the hell it is that plays in the DA2 trailers, especially at the end. That wonderfully inspiring track playing as Hawke stomps towards the camera, ready to kick ass? That alone should sell you on the game.
This troper would like to put it out there that This Is War is the god of all music and anyone who says otherwise is a liar.