Voice. Most powerful and natural instrument.

Natural? Yes, for obvious reasons. Powerful? Yes. Just listen to Azam Ali from 'Portals of Grace'.

I saw Rickie Lee Jones live back when she did 'Pirates' at the Pantages and her voice simply soared over the horn section like they weren't even there. She hit notes with such power that the whole place went slackjawed. No one could believe that someone who was still getting high back then and rambled on incoherently between songs (with one audience member yelling shut up and sing-she just laughed) could suddenly transform her little self into this vocal dynamo. It was quite a show.

Some performers can really belt it out.

After reading Elizabeth's response I think I'll take the Blue Whale over the current crop of diva's who simply yell as loud as they can. I could only watch two minutes of the Grammys and it was so nauseating to see all of them singing together trying to out diva each other. I was waiting to hear the gong but no one hit it.

I should also clarify that when some can really belt it out that the great ones do it without apparent strain. They don't bend over like they're having a seizure, mouth agape, eyes tightly shut, doing the Joe Cocker thing.
They simply sing way better than the rest of us.