I'm far too indecisive and have a much too broad range of music taste to decide on one person. I could listen to Johnny Cash all day and night I reckon though.. I also really like the sound of a lot of more modern male singers in bands like Billy Talent, Foo Fighters etc. and I'm a big fan of Alex Clare. Pavarotti is unbeaten as far as I know in terms of strength. My favourite female voices are probably Christina Aguilera, Beyonce, Whitney Houston, Aretha Franklin and Alicia Keys.. I like the sound of Lana Del Rey but I don't know how much of it is natural because apparently she's not very good live.
EDIT: Remembered a few more lol Frank Sinatra, Nina Simone, Etta James, Ella Fitzgerald and I've always liked the sound of Bryan Adams and Meatloaf