after reading some of the good answers which reminded me of some singers that didn't immediately come to mind and dreadful answers;
Soprano: Te Kanawa, Sutherland, and Tebaldi came in my head but for pure sheer vocal beauty it goes to Caballe
Mezzos: Graham, Larmore, Graves, von Otter, von Stade there are so many but my vote is for Rita Gorr
Tenors: Pavarotti, Schipa, Wunderlich, Simeneau ... in the end Tagliavini
Baritone: London, Warren, Merrill, Fischer-Diskau but in the end it must be Hampson
Bass: love Terfel, Ghiaurov, Ramey, Plishka, Morris, but I going for Siepi
I know I've left out so many greats (as far as tonal beauty goes) but this is what came to mind