For the most beautiful female voice I ever heard, Renata Tebaldi. This was a voice of glorious, Italian warmth that could wrap around you and make you melt.
Also on my list of favorites is the late, great Zinka Milanov who was for many years the Verdi soprano at the Met. The great Zinka was famous for "the note," a floated, pianissimo high B flat of ethereal beauty that carried to the highest reaches of the hall.
Of current divas, I love the dark, luster of Angela Gheorgiou's soprano. There's a quality that reminds me of Maria Callas. As a bonus, she has movie star physical beauty.