I think there's a difference between favorite (as in "best") singers and favorite artists. That is, if someone is a favorite singer he or she automatically is also a favorite artist. But someone who is a favorite artist may also be so because of different aspects of his or her artistry that goes beyond the the singing or the sound of his or her voice.
I'm a big opera lover and my favorite voice type has always been the soprano, and especially the lyric soprano. I can just wallow in the creamy beauty of voices like the ones from Renée Fleming and Kiri Te Kanawa. I also love Mirella Freni who for me is "the best" or most moving Puccini singer ever. Even late in life she was still utterly convincing singing roles of women forty years younger than her.
[video=youtube]http://www.youtube.com/watch?v=yD9q1y8Vv-s[/video]
[video=youtube]http://www.youtube.com/watch?v=trfLkYpOglA[/video]
Of the men I like Luciano Pavarotti, but only for his singing. As an opera "on stage actor" he was a disaster of the "park and bark" type. Basically a statue who's acting limited itself to opening his mouth and looking into the distance to express distress or smiling to show that he's happy or in love. Still, his voice was such an incredible instrument that it hardly matters. Rolando Villazon is my favorite young-ish tenor. His enthusiasm is almost impossible to resist, especially when he sings/plays opposite the super gorgeous (and great) Anna Netrebko. He's both moving and funny. There's definitely something Chaplin-esque about him. Placido Domingo is of course one of the all time greats. His longevity and his range of roles is simply incredible. I also love Nicolai Ghiaurov's bass voice. He's not with us anymore, but he was Mirella Freni's husband. But I could go on and on because the standard that is recquired to make it in opera is such there is no such thing as a poor opera singer who's made it - leaving aside a few crossover singers who are not "real" opera singers to begin with.
[video=youtube]http://www.youtube.com/watch?v=Pa58TdbRGJ4[/video]
I also love many jazz and "traditional pop" singers such as Billie Holiday (up to and including the 1940's, I'm not a big fan of the late "voice in shatters" version), Frank Sinatra, June Christy, Sarah Vaughan and Peggy Lee. Peggy Lee is one of the most undemonstrative "less is more" but nevertheless also sexy vocalists in any genre that I have ever heard. She oozes class and I simply adore her.
[video=youtube]http://www.youtube.com/watch?v=72h6Efv4l38[/video]
In more contemporary popular music I love soul singers such as Marvin Gaye, Aretha Franklin, Ray Charles and many others. Dusty Springfield is another one I love. I like her singing so much that I own all of her albums - even the mediocre ones just because I love that husky, yearning, sexy voice so much. In country music I like Patsy Cline (best country singer ever imo) and Alison Krauss.
[video=youtube]http://www.youtube.com/watch?v=CKXjYdrcmCw[/video]
[video=youtube]http://www.youtube.com/watch?v=kv3WSrZ-9bM[/video]
And then there's Linda Ronstadt - incredible voice and the most versatile singer I know. Rock, pop, country, bluegrass, mariachi, jazz standards, operetta - Ronstadt has done it all and she never embarrased herself with any of it. A few weeks ago I heard that Linda is suffering from Parkinsons desease and that we will never hear her sing again. Therefore I would like to close this post with a clip of of the incredible miss Ronstadt. Not any one video can do justice to her talent because she has done so many different things so well. But I haven't chosen one of the more obvious pop, rock or country hits that most will already be familiar with anyway. Instead I've gone for a bit of Mariachi music.
[video=youtube]http://www.youtube.com/watch?v=rJfXiOrK7t4[/video]
I've just realized that considering the clips I've chosen that I must have a strong preference for female singers. Perhaps I do, but there's plenty of guys I like as well though. Perhaps next time.