Several have said the "perfect song" is one you never get tired of hearing
Mine:
Spanish Moon--Little Feat
Asking Around For You--Joe Bonamassa
That's How Much I Feel: Ambrosia What? How the Heck did this sneak in here?
Going to California--Zeppelin
Old Love--Clapton
Once and Future Carpenter--Avett Brothers
Four Strong Winds--Neil Young
That's How i Got to Memphis--Tom T. Hall
I Was Young When I Left Home--Marcus Mumford's version
Free (Including violin intro) Zac Brown Band