You have a different definition of singer/vocalist to me. In my mind, vocals is the whole package... growling, screeching, clean singing, crooning, belching, ululating, pig squealing, African throat clicking etc... singing is just a subset of vocals.
Having said that, I think your idea of good singing is that the vocal melodies/lines are well written regardless of what the singer actually sounRAB like. But vocal melodies are definitely influenced by the music itself and whoever writes them (not necessarily the vocalist) - therefore, I don't think it's a relevant quality in compiling a list of great singers.