Even after pondering the question, I still think that Magdalena Kozena (born 1973) probably has the most beautiful voice around anywhere on stage today. And she just becomes better and better.
In my book, she is essentially a concert and lied singer, though she regularly performs in opera. Projecting mixure of shyness and awkwardness (not vocally mind you) on the operatic stage, I have seen her as both Octavian and Melisande. And her Idamante is available on DVD. However, it is on the concert stage, she does her finest work. And nothing she has done is really better than this recent disc with songs by Ravel (Sheherazade), Dvorak (Biblical Songs op. 99), and Mahler (Rückert-lieder) entitled "Love and longing" in accordance with recent record labelling trends.
With a creamy voice of incredible beauty, even throughout the register, she possesses an instrument as few (really is there anyone else?) today. Combined with profound textual understanding as well as faultless phrasing, I really do not have a critical word to say of this disc. Not to forget the superb accompanying provided by the Berlin Philharmonics under Simon Rattle. They just show us just how important the orchestra is in orchestral songs, with a performance far far above average.
The bottom line (scale of 1-5, 3=average):