1. Bob Dylan
Bob Dylan has a voice that seems to polarise opinion like few other musicians. In fact I know a few people who claim to love his music but can't listen to him because of his voice. However there is a good argument to be made that his voice is perfect for his songs, would he have been nearly as popular if he had a normal singing voice? I don't think he would. Dylan's songs have been covered by literally hundreds of other artists but how many of these cover versions have stood the test of time like the originals? Only perhaps Hendrix version of "All along the Watchtower", The Byrds "Mr Tambourine Man", Stevie Wonder "Blowing in the Wind" and a handful of others could it be argued that the cover eclipses the original. Nobody sounds like Dylan and his voice enhances his music.
2. Lou Reed
Lou Reed never had the most tuneful of voices when he first started with the Velvet Underground but got worse each year. I think he must have decided at some point during the early 80s, f$#k it I'll just talk the lyrics.
3. Tom Waits
Tom Waits makes Dylan sound like a choir boy. It's pretty incredible to listen to the transformation in Waits voice around the early 80s. When he first started out his voice was fairly normalish. Waits voice has an incredible potency to it, especially today. Whether it is a spoken word piece or a simple ballad, with Waits voice this music becomes something strange and otherworldly. Very few if any singers have the ability that Waits has to take a fairly simple song and make it sound so odd.
4. Neil Young
Neil Young has written some of the sweetest folksy songs and some of the greatest angry rock of the past 50 years. His voice seems at odds with these styles of music but somehow his high pitched warble fits perfectly with these styles. Songs like "Sugar Mountain" and "Harvest Moon" could sound like corny shtick in the hands of other musicians but Young seems to have an ability to make simple sentiments sound very meaningful. His voice has a fragility to it which is a perfect fit with the music he makes.
5. Kurt Cobain
Cobain had a unique voice. What he lacked in technical ability his voice more than made up for in raw power and aggression. Undoubtedly one of the all time great screamers in rock.