In the Wagnerian operas, two I believe are the most difficult - both for a DRAMATIC soprano: not a coloratura nor lyric.
These being Isolde in "Tristan und Isolde", and Brunnhilde in "Die Gotterdammerung".
The complete "Tristan" is I believe (uncut) a rounded 4 hrs. in length; that of "Die Gotterdammerung", a little over 4 1/2 hrs.
Principally what makes the Wagnerian roles so difficult ("hard") to sing, is the power a singer must possess to project and be heard over an approximate 110 piece orchestra: no easy task. And, ENDURANCE: Act I on "Die Gotterdammerung" is only 5 mins. short of 2 hrs. in length.
A You Tube video of the final scene of "Die Gotterdammerung": http://www.youtube.com/watch?v=BmX9N8C8nko
Birgit Nillsen - the most famous Wagnerian soprano of her day - was once asked by a journalist, "what's it take to sing the role of Isolde"? Her response, "comfortable shoes". This is a portion of the "Love Duet" from Act II of "Tristan": http://www.youtube.com/watch?v=ch3o0qV6TiA
Alberich