I've come up with a few of my favourites singers, who I happen to think sound have a TOTALLY unique and original style...
1. Tom Waits (absolutely unmistakable)
2. Stevie Wonder (much-imitated but no one gets close - not even the guy from Jamiroquai)
3. John Lennon
4. Joni Mitchell (particularly late-period)
5. Bob Dylan
6. Elton John
7. Michael Stipe
8. Van Morrison
9. Barry Gibb
10. James Taylor (not changed much in 30 years)
Any other suggestions?