'Long Distance' from Love Will Grow, the second FF Vocal Collections album. 'Pure Heart' is still a special song for me, too.
Out of the songs that are actually in the game... 'Melodies of Life' and the English version of '1000 Words'.

I'm deliberately leaving out the choral songs, though. It wouldn't be fair if I had to consider works like 'Liberi Fatali', 'One-Winged Angel' and the Hymn of the Fayth.