A perfect song? What are your choices?

I always thought of "Stairways to Heaven" as a perfect song.

It has everything I found necessary in a (rock) song - slow part, great melody, powerful lyrics, strong verses, solos...

And most importantly, geniuses behind every instrument and Robert Plant's voice.

I wouldn't say it's perfect but I've literally played Cat Stevens' Portobello Road hundreds of times.

If we are limiting it to Cat Stevens, I vote for Sad Lisa.  Maybe even if we don't limit it to Yusuf.  

sometimes people are born to whom songs are dedicated ... By the way, whoever has not seen it, I recommend the autobiographical film "The Diary of a Motorcyclist" (2004).