I strongly disagree. A voice is so limited in every single way. There is so much more that can be done with a proper musical instrument, especially those capable of multiple simultaneous notes. The piano is a good example.
You can always have multiple people singing if you don't count that as cheating. If limits are the deciding factor of an instrument's ultimateness, then I raise that a computer with quality sound hardware + hard drives full of real world audio samples + programmable oscilators is the ultimate instrument.
Now that I played my role as captain obvious, fuck it, piano is really the best! (No sarcasm)
I strongly disagree. A voice is so limited in every single way. There is so much more that can be done with a proper musical instrument, especially those capable of multiple simultaneous notes. The piano is a good example.