Rolling Stone asked members of a "blue-ribbon panel" to name their favorite vocalists, and from their responses, it compiled its list of the 100 Greatest Singers Of All Time. Top of the list? Aretha Franklin.
It's an interesting list, and provided you understand that it's not terribly broad (as commenters quickly noted, there are no great vocalists who sing primarily in other languages?), it makes a nice, compact collection of mighty vocalists.
Be warned: They show a "playlist" for each artist, and it's very nice that they went to the trouble of setting it up, but it isn't ordinary streaming — it plays through the Rhapsody music service. Rhapsody will give you 25 free streams a month, but you have to install their software, so think of the playlist as a nice idea more than a functional add-on for the average surfer.