I've encountered another song, that gives me more criteria to judge a song by. From an album that probably aims higher than any other in the thread. In the words of the singer, the aim was to bring poets from long ago to life, as though they are conversing with you face to face. I mentioned above that I think the lyrics should stand alone in quality, even before you consider its combination with the music. The words are hundreds of years old, in this case over a thousand years old. The lyricists are regarded as greats, in this instance
the seminal figure in the development of Chinese lyric poetry. This particular poem speaks of loss, reflecting on the poet's loss of his kingdom (he was a former monarch now held prisoner in the enemy capital). The tune and the singer absolutely nails the poet's emotion, with the poet speaking from over a thousand years ago through the singer.
The song moves me, but an objective assessment should not depend on my personal reaction. By just about every objective measure, this song is perfect or as close as it can get.
Du shang xi lou (Alone in the west tower)
Singer: Teresa Teng (recorded 1983)
Lyrics: Li Yu (written 975-978)
https://www.youtube.com/v/CQyjJj-ws84