Excellence in everything. Excellent high range/pitch/tone singing, melody, energy, progressiveness/technicality/time changes which helps to keep it interesting and not repetitive, exotic influences demonstrating wide grasp of musical understanding (classical, folk, opera, world, spanish, middle eastern, indian, arabic, jazz, asian, etc) heaviness, uniqueness. Creative Academic/Academy quality level, aim, and production is almost always my thing in art/music or whatever.
It can be slower, middle speed/energy, heavy, or really heavy, as long as it's done well. If it has death or gritty vocals it should be balanced with clean vocals, but either way they must leave a good impression, passable is passable, pretty good is pretty good.
Though I like some mid range singers as well,but I rarely like lower range, though IF they're just so cool, full of character, or unique and good in their own right, and can pull it off then great. I can't take a lot of it but it's also nice to listen to. I definitely preffer mid range register singers to lower range any day, but if there a choice between only those two I'll most likely choose mid range. The less their singing is like talking, and more like singing (ie higher, or even some mid) then coolbeans. There are few death/black metal voices that I like, Chuck Shuldiner is one of them, Dimmu, and At the Gates are some others.
My Myspace page shows examples of what I consider to fullfill this kind of criteria in terms of what I'm looking for in music. If a singer can do his/her own voice AND other styles of singing that can be really cool. I think some of the best vocalists are Andre Matos of Angra/Shamaan etc, Toshi formerly from X-Japan, Vitor from Aquaria, Michael from Circus Maximus, Tarja of Nightwish, Jim Foss of Sea of Dreams, Fabio of Rhapsody, Lance formerly of Pyramaze, Olaf of Luca Turilli, Stu of Into Eternity, Sharon of Within Temptation, Simmone of Epica etc..
Bottom line I guess in terms of what is most important is excellence in singing and excellence in the composition of the song writing. I love mind blowing genius, and I think that is demonstrated by having a good control over a balance of the theoretical and the emotional in the songs they write or produce.
uke: 
