Anna ni Isshoni Datta no ni, Fields of Hope, the second song by Lacus in GS (don't know the title, sadly) and Reason are all awesome songs. Ignited is pretty good, as well.
EDIT: A couple of extras I forgot: the song that they played during Shinn and Stellar's first meeting, and the song played during Athrun and Cagalli's first meeting were both awesome.