Chris Cornell, Chester Bennington and Freddie Mercury.
When you are doing your level best to sing along to anything they have graced (whether at home, in the Car etc).
You instantly know just how talented they were, how much range they had and how much their performances lift songs to another level.
Others on my list who, while not necessarily the most proficient, have a voice that just marries perfectly with the band they are in, or style of music or it just has a uniqueness that just appeals to me: Michael Stipe, Billy Joe Armstrong, Brandon Boyd, Tom Delonge, Eddie Vedder