Eyes On Me and Melodies of Life are the only two that I considered in the poll as the others feel like they were chosen and placed into the game rather than having a significant story.

In Final Fantasy X-2, the vocal themes were placed in the story without any significance than being a part of a performance for the background characters. In Final Fantasy VIII, Eyes on Me was a song that Julia wrote about Laguna, but was not actually heard until much later in the game. In Final Fantasy IX, Garnet would sing parts of Melodies of Life occasionally.

I vastly prefer a song with importance rather than a song chosen to represent or symbolize a part of the game or the entire game. My Hands by Leona Lewis, for example — what does it have to do with Final Fantasy XIII besides being a pretty song that plays during the credits roll?