Though I can go only by their recordings, and didn't have the pleasure of seeing them in concert, two choices for me would Eva Cassidy and Janis Joplin. They were expressive, powerful and soulful. Actually, my list on this subject is endless. However, as previously indicated by Jerry, one doesn't have to have a so called good voice in order to move me. A couple of examples would John Prine and Mark Knopfler. Besides their writing, interpretation and phrasing are their strengths. This really begs the question of what makes a great singer. That's a thread for another day.