Just through my own listening experiences, here's five vocalists I could never ever mistake for someone else:
Karen Carpenter
(During Christmastime a lot of businesses play music. When I'm with my youngest sister I'm notorious for stopping dead in my tracks and just saying, "I hear Karen!" - she knows who I'm talking about immediately.)
Ric Ocasek
(If I could have "scrobbled" my entire childhood, the amount of times I listened to The Cars would completely surpass everything else. Despite the fact that lead vocals were also shared with Benjamin Orr, the fine Mr. Ocasek is simply unmistakable. He's like what would happen if Buddy Holly and Iggy Pop would have been born one in the same.)
Pat Benatar
(As I once heard a radio DJ say, she's "five feet, three inches of rock fury". Incomparable badass lady royalty.)
Morrissey
(Despite the fact that I haven't always been into his music, with or without the Smiths, Moz has this voice that's just so freaking weird, wonderful, slightly difficult to really get into, but unique enough to keep me listening.)
Russell Mael
(The only American that can easily pretend to be British. Not exactly a father of falsetto, but the man has pipes that could cause a person to spontaneously combust - or faint!)
Females that come to mind:
Leslie Gore
I love Lesley.
For some reason her voice basically fills a void in my soul.
It's funny because I actually was going to write in Raine Maida as well for his distinctive vocals! 
