I don't really know how to define a perfect song haha but I have one song that definitely sits there for me and that would be Octavarium by Dream Theater.
After that, I feel like a perfect song is one that I just really think is perfectly written, performed and produced that I will always enjoy listening to any time. I have probably quite a few songs that fit this category.
Lie to Me - Johnny Lang, I don't know what it is about this song, but it's just great to listen to.
Time is Your Friend - Dada, thanks to Kingsy I discovered this song and it's just amazing. One of the best choruses ever.
Deadhead - Devin Townsed, just an incredibly well written and performed song. The vocals are very moving.
I can't think of any others off the top of my head right this second.