Agree on the rock side. It's a fine line between rock and pop to me. I like Stevie's voice the best, it comes across on record the best.
I like Susanna Hoffs on the pop side. Lisa Gerrard is my all time fav in terms of emotional connection with a singer, of all of them.
For distinctive voice...does it have to be a classically appealing voice? Dudes like Lou Reed and Peter Murphy have a crazy unique voice. Tough one. I like Al Green as a safe pick. My Family Feud pick would be Robert Plant, but folks might say Dylan or Waits.