Harry Nilsson was one of the greatest singers in rock history, a favourite of John Lennon, Paul McCartney and David Bowie. His 1971 album Nilsson Schmilsson is the place to start for someone not familiar with his work (which would apparently include the people who voted on Rolling Stone's 100 Greatest Singers list, from which Nilsson, ridiculously, was left off). Both the album title and its famous cover photo (http://en.wikipedia.org/wiki/Nilsson_Schmilsson) demonstrate his self-deprecating quality, but don't be fooled, the album contains some of the most wide-ranging, adventurous and virtuosic rock singing ever done. The big hits, "Coconut" and "Without You", both demonstrate the vocal variety of which Nilsson was capable, but to really get a sense of his artistry, I'd recommend listening to the entire album (and then some others after that). Here's the first track, "Gotta Get Up", to get you started: http://www.youtube.com/watch?v=IKy_gTrdXaU.