[Woodduck]

It may be difficult for anyone not a singer to answer this definitively, but I would say it would depend on the singer as much as the role. Lilli Lehmann famously said she would rather sing all three Brunnhildes than one Norma, but I suspect Joan Sutherland would have felt differently. Either part might be a candidate for most difficult female role, with Isolde very much in the running. For tenors, Tannhauser, the Siegfried Siegfried, and Tristan are all notorious killers, but I imagine Otello is a close challenger. Staying with Wagner, the Walkure Wotan requires just about everything, vocally and dramatically, a bass-baritone can give.

 

[Woodduck]

Lord Harewood knows a thing or two about singing.

Yet, when he was interviewing Callas, he mentioned that bel canto was "harder in some ways" than Wagner. Callas without pausing added, "in EVERY way."

I love a Wagnerian Hochdramatischer Sopran in her prime like Kirsten Flagstad, but I just don't see it in the cards that she'd have the agility to sing Norma the way Callas did in her prime.

Callas could sing Isolde compellingly but the reverse is not true with Flagstad being able to sing Norma.

Flagstad was offered Norma, considered (and probably practiced) it, but wisely decided against it. When the role was suggested to Nilsson, she said, also wisely, "too many small notes." It's interesting that the part should have been suggested to two such huge-voiced singers of different generations, indicating that it has long been regarded as a dramatic soprano role, but one calling for exceptional agility.

 

[Woodduck]

Tristan and Isolde in Wagner's opera are probably the most difficult roles ever.

For these roles sheer stamina is a prime factor. Singers literally run out of voice before the opera is over, with unfortunate results for some of the most dramatic and beautiful music in the work and an ending that should be cathartic and transfigured. It's the main reason why the opera has so often been cut; the traditional cuts are in two places, chunks being taken out of Act 2 and the middle section of Tristan's tripartite Act 3 delirium. In the production I saw at the Met, the cuts were made and Helge Brilioth ran out of voice anyway, after which the redoubtable Birgit swept in and brought down the house with the "Liebestod," sounding fresher than she had at the start of the opera.

I don't think anyone should underestimate the strain of these roles properly performed. The requisite combination of sustained dramatic intensity and sheer power needed to sound vocally firm and fresh against heavy orchestration throughout a long evening is rarely supplied in full, even by the most heroic-voiced singers. Tristan is widely regarded by tenors as the ultimate mountain and a late-in-life project, when one's voice, endurance, and sense of pacing is fully developed and there is less to be risked in terms of sabotaging one's career. There also seems to be a "deceptive simplicity" factor with Wagner, pace Callas: the obvious technical difficulties of a role like Norma will keep many singers away from it, and those that do tackle it will soon have their weaknesses exposed; but singers who imagine they can handle Wagner's heroic parts may have some success with them and go on singing them for some time while their vocal facility and beauty gradually deteriorate under the strain. The notes themselves may be easy compared to Bellini or Handel, but the assiduous practice of bel canto technique is healthy for the voice in a way that constantly pitting your middle register against Wagner's orchestra is not.

 

[Woodduck]

I think this extends to Mozart in general: whose works are, imo, an odd combination of vivid characterization but vocal lines which feel somewhat mechanical and un-intuitive on the voice.

Unlike Puccini, who writes big soaring lines and has the orchestra double them, Mozart leaves you on your own. He will not help you sound good if you aren't. His vocal writing requires perfect poise and technical discipline, and if you lack it everyone will hear by just how much you lack it. Singers know this; "practice your Mozart" means "mind your technique, find your weaknesses, fix them." It also means "don't force, keep it light, keep it flexible." Singers who specialize in Verdi or Wagner should practice their Mozart, even if they can't sing him well enough to perform him in public. Birgit Nilsson could never have sung the Queen of the Night successfully, but she sang her arias at parties, complete with high Fs.