Hardest to SING vocally/technically:
One of the hardest to cast overall is Bellini's I Puritani because it requires 4 great technical singers, also his Norma.
For soprano voice I would say Verdi's Attila, Nabucco, Macbeth, Il Trovatore, Rigoletto, La Traviata.
Rossini's Armida, Massenet's Esclarmonde, Meyerbeer's Huguenots, Delibes' Lakme, Tales of Hoffman if you cast the same soprano for all 3 major characters, Mozart's Magic Flute because of the Queen of the Night, Mozart's Abduction from the Seraglio because of Constanze, Donizetti's Anna Bolena, Lucrezia Borgia, and Elizabeth in Robert Devereaux are extremely difficult to sing.

 

Of the major works in the repertoire the easiest to sing basically vocally/technically: Mozart's Marriage of Figaro. Also lots of non-virtuosic obscure baroque operas.

 

Purcell's Dido sounds fairly simple and was written for a girl's school. (Most typical high baroque operas were written for supervirtuoso castrati and divas).

Then there are operas written explicitly for children like Krasa's "Brundibar" and Menotti's "Help, help, the Globolinks" (although when I saw the latter, I think the protagonist was a young professional, IIRC she has to be able to play the violin as well).

 

Dido was the opera my sister's college performed and she was Dido. It is not demanding, but it the hands of a great interpreter it can be sublime art.
The most difficult for sopranos: Elektra, Brunhilde the Ring, Isolde, Semiramide, Armida, Esclarmonde, Norma, Abigaille, Leonore in Trovatore

 

If you have no technique (like me) then every operas is hard.

 

Handel can be extremely taxing on singers, particularly sopranos and altos, but the music he wrote for great bassos such as Giuseppe Maria Boschi and Antonio Montagnana is also very challenging. One reason is that his superstar singers and their fans demanded virtuosic arias to show off their abilities and Handel was usually happy to oblige. To take just one example, Handel’s Alcina is extremely hard and very challenging for the singers. It takes a lot of focus and concentration. Professional Handel singers these days usually add ornamentation –more or less appropriate – to the da capo sections of arias, although very few yet do as their 18th-century predecessors did and actually improvise – or, more accurately, compose – on the spot, new melodic material in the repeats.

 

Most of Rossini’s tenor roles are difficult, particularly Arnold. Bellini’s Arturo too.
For sopranos, Cherubini’s Medea, Bellini’s Norma and Imogene, Donizetti’s Anna Bolena, Rossini’s Armida and Ermione
Verdi’s roles are probably the hardest for baritones and mezzos. Donizetti’s Leonora too for mezzos.

 

Vinci's Artaserse - only one recording, and a great one - has virtuosic countertenor singing that leaves me totally entranced and awed.

 

I wish we had some recordings of operas like this with idiomatic singing. As impressive as the florid singing may be, the style of vocal production of singers like Jaroussky makes it very difficult for me to take these accounts seriously.

 

I always thought learning and then singing Wozzeck has got to be one tough problem. Any 12-tone opera for that matter.

 

Difficult listening as well

 

I always loved music and at school, I was in a little orchestra or chorus, where I was not allowed to sing due to my pitch processing impairment. I was assigned the role of the moderator. Anyway, I dimly recollect we had a children's opera in our repertoire. I assume it was easy

 

I’d say Händel‘s Giulio Cesare in most of the parts is not a walk in the park, due to the extreme ornamentation and variations thereof in all of the arias for Cleopatra, Cesare, Sesto, Tolomeo.

That said, no other opera by that composer would be easy! Operas written for castrato singers were composed to show their virtuosity, breath control, coloratura, extreme high and low notes, beauty of voice and alacrity, not to mention musicianship.

 

That's why I trumpeted Vinci's Artaserse, where you've got five countertenors and one tenor giving this most taxing of performances imaginable that I've ever heard and in the most beautiful way, and the music of this composer is first-rate as well. Vinci didn't have long to live - he was poisoned three months after the premiere, and died at the young age of ~33.