Just singing voice? What about actors? They count as artists. I like Clancy Brown's deep voice. It can be very intimidating and scary (The Kurgan) or very funny (Mr. Krabs). John Malkovich is another actor that has a very interesting voice, or at least he does to me. As for musicians, I've always like Axle Rose's screeching pitches, and that sound I can't even describe from Johhny Cash's unique voice. There are probably a lot more I like, but those were the first ones the came to mind.