I don't know about perfect song, there is no such thing IMO, but two I really enjoy singing and regularly come back to are

The Three Ravens
Jock of Hazeldean

In both cases it is a combination of the story, the melody - both have fine melodies - and the images that are conjured up by the song that give me a buzz when I am singing them.

I sing both unaccompanied. Just a matter of finding an comfortable key to sing in.