And by song I mean original song with lyrics and vocals. It has to have lyrics. And it has to be originally composed for the game in question.

Personally, I don't think anything can surpass Radical Dreamers ~The Unstolen Jewel~ from Chrono Cross. As meh as I think most of Yasunori Mitsuda's songs with lyrics are (let's face it; they're all basically reworked versions of "My Heart Will Go On"), this one is really unique.