I am fond of a lot of female musicians for a variety of reasons, but to elevate that status to crush level takes a special sort of combination of beauty, talent, sex appeal, etc. In no particular order.
Nancy Wilson (Heart, guitarist) - picture in Dreamboat Annie did it for me. And she was sort of the "strong silent" type (although not completely), so I kind of related to her (I'm more of a multi-instrumentalist than a singer).
Shirley Manson (Garbage) - from the first time I saw them live, too. She's married, though (but not to anyone in the band -- some sex toy sculpturist...actually, that sounds wrong, that sounds like he sculpts sex toys, that's not what I meant; what I meant was, he's a sculpturist, and he's her sex toy).
(Like I had a chance
Kate Bush - I was also a fan of this lady, but what took my adolescent hormones over the edge was the infamous picture of her in a leotard. Her sex-kitten and overcoat performances on SNL didn't hurt, either.
Jennifer Love Hewitt - I was actually familiar with her from her self-titled album's cover before her work as an actor. "Those soft and fuzzy sweaters, too magical to touch..."
...but hands down, number one, is Milla Jovovich (Milla, Plastic Has Memory) - I've developed a crush on her several times, without even knowing it's the same person -- she's a supermodel, she's an actress, she's a musician, what can't she do? Still has a special place in my heart. I really wish she would get her act together and put an album out (as PHM).
PS On Mariah Carey's dumbness -- that's a farce. I know she acts like trailer trash, but I actually admire her for divorcing her Svengali husband and doing her own thing. Butterfly was sort of her "Never For Ever".
On the brink (waiting for something to send me over into crush-hood):
Tori Amos
Anneke von Gsomething (The Gathering)
Heather Thompson (Tapping The Vein -- amazing voice, quickly becoming one of my favorites, from Kate Bush girly coo to Drain STH, node-blowing, full-out rock'n'roll bombast)
for that matter, the bassist from Drain STH
