Freddie Mercury was the first person on my mind when I saw the thread title.
He'll always be the best for me. There is no doubt how great a singer he is, so there is not much to explain. And then next up will be
Vitas. He is a singer from Russia, and I love his unique, high pitch voice. The sad thing is, he seems to lip-sync quite frequently = = I'd only seen a couple of his actual live-singing performances on youtube, and they are all amazing.
A-mei Zhang is also a great singer. I don't know if this sounds right, but her voice feels so "clean" to me. I can't find anyone in the music industry with a voice as "clean" as hers.