A few "most distinctive rock voices" lists have popped up, but are comprised almost entirely of male voices, which seems odd to me. I tend to describe my music tastes as preferring shouty-girl-rock to whiny-boy-rock, largely because the vocals are often significantly more diverse and interesting.
So, my list. Criteria were: defining "rock" as primarily guitar-based music that changes between fast and slow (which led me to leave out Tracy Chapman, Joni Mitchell, and Amanda Palmer, amongst others), and distinctive voice as being more than simply a famous female vocalist.
I've also tried to put together a decent mix of the old and the new.
Seminal indie rock band Sleater-Kinney is known for many things, as well it should be: driving rhythms, melodic interplay, no bassist. But its most notable attribute is lead singer Corin Tucker's piercing wail. It drives its emotion right through the listener, no compromises.
Debbie Harry is similar to Mick Jagger in a certain respect as singers. Their voices, in and of themselves, aren't terribly special. But they bring a certain swagger, an indelible way of saying "I'm a fucking rock star" to their performances.
Two double threats here. First, Kathleen Hanna of classic riot grrl act Bikini Kill and electro-pop outfit Le Tigre. Hanna doesn't necessarily have a distinctive voice inherently - but her presentation makes it stand out. In Bikini Kill, like any good punk, she turns her lack of traditional singing chops into a nasal whine of rebellion.
With the much poppier Le Tigre, Hanna still turns her voice's weakness into a strength, hitting the high pitches in insanely catchy fashion.
Case's double threat is even more diverse than Hanna's. In her mostly solo output, she does alt-country with a rock bent, while she's also a singer with the superb power pop group The New Pornographers. She's got a voice that can hit amazing twangs as easily and beautifully as she can do Beatles-esque harmonies.
It would be hard to do a list like this without Springfield, especially after having Joplin at the top. Both Joplin and Springfield have husky blues-rock voices, but where Joplin wails like she's leaving everything behind after this song, Springfield brings the emotion with a restraint that always sounds like it's about to break.
I've heard Chan Marshall (aka Chan Marshall) called the model for an entire generation of female indie rock singers. This seems slightly overrated to me, but there's no denying the impression of her soulful, near-breakdown voice.