Voice. Most powerful and natural instrument.

Natural, yes. Powerful, certainly not.

Emotionally powerful? Yes.

Natural...well, that goes without saying. A primary goal of musicians, particularly players of wind instruments and strings, is to replicate the emotion of the human voice in their playing.

Emotionally, of course. You can do rhythm with it quite good too.

The voice is usually the most important instrument in the mix

Try and imagine the Doors without Jim Morrison.

Voice can, rarely does, but can carry the show. Is opera natural singing? If so what some, say, Placido Domingo can/could do is enough to move you just by the voice.

How many great songs have been made by the particular singer and to such an extent that for a generation or more they have the definitive version. Nat "King" Cole and the Christmas Song.

The voice may not be the most powerful or natural but it is the most magical as to what it can do.