Even though my favorite genre of metal probably is black metal, I will NEVER find someone that sings great in that genre. I never really liked the vocals in black metal, but I've learned to live with it and they don't bother me anymore. The "better" ones (or least worse) of black metal vocals I think is Shagrath from Dimmu Borgir, the guy from Dark Funeral (can't remember the name) and Satyr from Satyricon. The worst black metal vocal I've heard is Dani from Cradle of Filth. I don't know if it has changed, but the vocal on "Dusk and her embrace" and "Vempire" is probably the worst vocal I've ever heard!!! Varg Vikernes' (Burzum) old vocal (before "Filosofem") is also really bad! Even though "Hvis lyset tar oss" is my favorite black metal record of all time I hate the vocal, but I've learned to live with it.
In the better end of mediocre female voices I think Aaliyah and Heather Nova fits in. I've grown a little tired of Aaliyah's music, but I still think she has a decent voice. Listen to the verse on "More than a woman"! I haven't heard much of Heather Nova's music and what I've heard was just mediocre, but I also think she has a decent voice. I also have an addition to great male singers and it's Axl Rose from Guns N' Roses. Basically he's a terrible singer, but his voice fits their music perfectly! When I've heard him sing a song with another singer (which wasn't a GN'R song) it sounded terrible and also the live performances I've heard by them sounded rather crappy. His perfomances on record does, as I said match their music perfectly and I would be amazed if someone could replicate his job on songs like "Nightrain" (from "Apettite from destruction") especially after the guitar solo, "My michelle" and actually everything else. I played in a band where we only played GN'R and the only song we were able to play was "Mr. Brownstone" as our singer couldn't hit the notes Axl did in all the other songs. Our singer sang pretty good and I heard someone trying to copy us and it sounded terrible when they played "Paradise city"!!!! Axl's range is pretty incredible. If I try to hit the same notes without singing in falsetto it's nearly impossible or it will sound like shit!
There are someone who sings like shit. Dido is one of them. If she doesn't try to hit high notes and sing to loud her voice sounds okay, but when she does these two things it sounds like pure shit!!!! Especially in that single where the video is from a live show. Also her performance at the last European MTV music awards where she sang that song "Hunter". She sang completely out of tune and it was horrible!!! Mariah Carey is often refered to as a great singer, but I think her singing in recent years has been terrible. If you listen to her earlier work from the early 90's she sings very different than she does now. Now she sings similar on all her songs and it gets tiresome after 30 seconds. It's hard to explain how it sound, but try and listen to for instance "Emotions" and then listen to "Heartbreaker" and hear the difference in her voice. You can listen to them at Cdnow.com. "Emotions" is on "Emotions" and "Heartbreaker" is on "Rainbow". The first one is from '91 and the second one is from '99. Her music sucks though