That may seem like an impossible question to answer, and yet people do ask this question. I believe that the human voice is probably the most powerful instrument because of the infinite gradations of emotion that can be expressed with the voice. When you add the element of acting and facial expression and bodily gestures, true emotional power is definitely possible!
Recently is was on vacation in Cabo San Lucas, Mexico. While there I read a wonderful biography of the opera star, Maria Callas. I highly recommend that you read this book which is by Arianna Huffington. Callas was a Greek opera star who had a difficult childhood (like so many of us!) but was able to use her painful experiences to bring dozens of opera’s heroines to life with an unbelievable emotional power.
Of course there are many instruments such as violin and cello that can also express powerful emotion, but overall, I believe there is nothing more powerful than the human voice! Enjoy this fantastic and beautiful rendition of “Ave Maria,” sung by Maria Callas:
If this doesn’t put you in a mood of reverance and humility, I don’t know what will!