So back in the mid 90's, my high school girlfriend Sara and I went out and bought the new double-disc CD from the Smashing Pumpkins, Mellon Collie and the Infinite Sadness. In the olden days, we had to listen for songs on the radio, then if we liked it, we went out and bought the album the day it came out, assuming you could find it. Anyway, so the song '1979' came on and when it was over Sara turned to me and said "that is the perfect song." (That was a great comment to make but you should note that Sara turned out to be pure evil later on in life, so it didn't work out)
So recently I heard that song and remembered that comment and thought 'she's totally right.' So what's your idea of a perfect song? Does it have to do with lyrics, music, mixing, structure, what? I suppose you don't even have to like it for it to be your idea of a perfect song. For example, Jagged Little Pill by Alanis Morissette is very well respected for it's sound engineering. Not particularly my ball of meat, but one could have a track on there that is their perfect song. The song that carries out the idea perfectly and there is no way they could have done it better.