Voice. Most powerful and natural instrument.

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.

Handel probably more than any other opera composer I've heard used the voice as an instrument, competing and complementing other instruments in their settings, especially in Julius Caesar.