Caveat: I'm no singer!

I think that a really good song sung well, which contains really good, tasteful instrumental work played well, is a single entity, not words with bolted-on accompaniment. The instrumental work doesn't have to be simple either. Have a listen to Sixteen Come Next Sunday by the Bothy Band on YouTube. The instrumental work is fantastic, ever-building to that wonderful unresolved finish, yet nothing takes away from the story. Good taste, a lack of ego, no sign of strain, and consummate team spirit. My favourite singer-guitarist is Nic Jones. To me, his voice and guitar on Noah's Ark Trap and Penguin Eggs are of a piece, utterly inseparable. Another great guy is Andy Irvine. I could go on, but I am but a small voice in this thread replete with experts!