A good sign is when you read the thread title and songs pop into your head immediately... If you have to think about it too long, then there's a chance the song isn't as perfect as you'd think.
For me?
Neko Case - "Hold On, Hold On" from Fox Confessor Brings the Flood
Opeth - "When" from My Arms, Your Hearse
I'm guessing there are other's I'd think are qualified, but that would take time for me to think about it, and that's something I've been short of lately.