The human voice is the original musical instrument, and humans have evolved to specifically listen to the human voice. I love the piano, but I don't think it is a stretch to say that for humans, the human voice is the ultimate instrument.

I don't mean to sound pretentious, but if you just look at the popularity of different instruments, voice is the runaway favorite. The majority of the world listens to mostly voice as the primary instrument.

To call it the ultimate instrument is not a stretch at all since it is the oldest and most popular instrument in human history.