The most demanding roles for Soprano are
the Assoluta roles:
1. Norma: it's been already established on this thread. I have nothing to add.
2. Abigaille in Nabucco: The ultimate Dramatic Coloratura challenge. An epitome of Verdi's real and rarely-met demands, it calls for as much Volume as Wagner, as well all the Bel Canto virtuosity and agility. A wit called early Verdi "Bel Canto on Steroid" and it's perfectly true.
The role is replete with treacherous scales and other crazy feats that only Callas could do justice. She made the roles even more exciting even interpolating a high Eb.
The stupid plot requires great acting skills as well to make it at least plausible. Abigaille should look and sound young, yet fierce and regal.
3. Lady Macbeth: Same as Abigaille but much more interesting and mature dramatically and only slightly more forgiving vocally. My personal favorite female role. The gran scena del sonnambulismo is one of Verdi's toughest challenges, not only musically but dramatically. In the same interview in which Callas explains why she thinks Wagner is "so easy" (for her at least) she also dissected this scene and talked about the nuances and colors she used for every single passage and why she used it.
4. Medea: Norma minus the fioriture plus more wickeness.
Other difficult but underestimated
Verdi roles:
- Leonora in Il Trovatore: this is a long role, calls for incredible vocal beauty, tons of ornaments (trills, mezzo-trilli, trills in a slow melodic line, staccati etc..) as well as a very solid lower register. Miserere is Dramatic Mezzo material. It is followed by a very difficult Dramatic Coloratura cabaletta which is often dropped. D'amor sull'ali rosee is "perhaps Verdi's ultimate challenge". Besides, the acting has to be so subtle and convey the noble character of Leonora which I've only seen Kabaivanska do so well.
- Elena in Vespri Sciliani
- Elizabeth de Valois in Don Carlo
What makes
Wagner difficult is indeed the Stamina it requires. I remember Dolora Zajick saying Verdi kills you in two hours but Wagner gives you three or four hours to die or something to that effect.
Turandot is not difficult if you have the right voice. You only need to sound like trumpet. Nilsson did. That's why she was the best and she called it her "vacation role".
Donizetti's three queens are somewhat underestimated in difficulty, especially Elizabeth in Roberto Devereux and Anna Bolena.