In general mezzo-sopranos play guys, mothers and sluts. Since she is tall, Susan Graham specializes in guys. By now I have collected quite a few of her videos, and she sets a standard in this category. The mother parts are usually minor.
There are two categories of roles where mezzo-sopranos play men: those originally intended to be sung by castrati and those intended to represent adolescent boys and originally sung by women. A countertenor may reasonably stand in for a castrato in any context without violating my sensibilities. Roles intended to be sung by women should be sung by women.
1. Gluck's Orfeo ed Euridice .
Chronologically the list starts here. The title role of Orfeo is composed for a mezzo castrato and was transformed into a tenor for the French version. Only sometimes is this revived for a mezzo. In school one is taught that this is significant as the first reform opera, an opera that "reforms" the extremely formal Italian opera seria of the era. A reform opera is a serious opera without da capo arias and secco recitative, but no gaps appear in the production of opera seria because of the appearance of this opera. The true significance of Orfeo lies in the foundation of the French grand opera.
I suppose there are quite a few mezzo and contralto castrato roles in opera seria, but I haven't listed any of them in my top ten.
2. Rossini's Barber of Seville.
When it was originally presented, this opera was a scandal, because it was composed to the same libretto as the already beloved opera by Paisiello, which it eventually replaced in the repertoire. The heroine, Rosina, is a coloratura mezzo, but the role is sometimes sung by a soprano with a lot of interpolated high notes. Rosina is the ingenue and not a slut.
3. Rossini's La Cenerentola.
Angiolina, the heroine of La Cenerentola, is a coloratura contralto, and is generally revived only for mezzos or contraltos. She has no big arias until the very end of the opera. Angiolina is not a slut either. Perhaps the stereotyping of mezzos began later.
4. Rossini's L'Italiana in Algeri.
Isabella is also a coloratura contralto. Isabella is in a harem, so it is difficult to decide if she is a slut. Let's just say that she is a sophisticated young woman. Rossini was fascinated by the comic possibilities of interaction between the sexual freedom of Italian women and the more confined expectations of Islam. That's as polite as I can say it. What would an Italian woman who found herself in a harem do? This opera answers that question.
4.1. Rossini's Il Turco in Italia.
This doesn't really make the top ten list, but Fiorilla is definitely a slut, perhaps the prototype for all the other great slut mezzo-sopranos. This opera answers the question what would an eastern man do when surrounded by an entire nation of Italian women. He wastes no time.
5. Donizetti's La Favorite.
Why did I put this in? Probably because I always wanted to sing it. Leonora is a coloratura mezzo with a lot of melodrama in her singing. This opera was originally in French, but is now more commonly revived in Italian. She is definitely a slut of a rather refined sort.
6. Bizet's Carmen.
We are finished with the bel canto tradition and have moved into the romantic mezzo style. These next three operas are all in the French tradition and form the meat of big roles for mezzo-soprano.
Carmen is an actress's dream, the great slut of the opera world. I now feel that Carmen is looking for a spy inside the police department to help cover her smuggling operation and deliberately sets out to seduce Don Jose as the most likely (and also attractive?) candidate.
7. Saint-Saëns' Samson and Delilah.
Dalila is definitely second only to Carmen in the hierarchy of great slut roles. We will give her points for being a political slut, I suppose. Where Carmen is audatious, Dalila is seductive.
8. Massenet's Werther.
Charlotte is an aberration in mezzo repertoire: she is a good girl, someone who would normally be played by a lyric soprano. How did this happen?
9. Tchaikovsky's The Maid of Orleans.
I just heard this, but it was so spectacular that I had to add it to the list. This role is for a heavy dramatic mezzo who also gets to sing some beautiful lyric passages.
10. Strauss' Der Rosenkavalier.
The title role is for a mezzo. It is the most significant of all the trouser roles and is always sung by a woman. If someone is doing it with falsettists, I don't want to hear about it. Octavian can be upstaged by the woman who plays the Marschallin, but the mezzo can dominate the opera if she wants.
So what's on your list?