The only song that EVER made me stand still and listen, with my mouth hanging open in wonder, was 'Bridge Over Troubled Water'.

It has the perfect combination of a beautiful lyric, haunting tune, strong emotion and wonderful vocal. Despite the fact that Paul Simon later admitted that the final verse doesn't really fit (it's true, it doesn't).

Other than that, I'd say there are 3 possible reasons why a song might be perfect:

1. You wrote it yourself (if you happen to be a great songwriter),
2. It brings back strong memories of that certain moment in your life, or
3. There's just something about the melody/words/message that you love; and this will be different for all of us, thank goodness!