Greatest Opera I've Never Seen?

After I saw Les Troyens for the first time in 2015, I felt a twinge of sadness, because at that point, I'd seen most of the (allegedly) greatest operas in the repertory. Recently, I've been contemplating what, at this point, is the greatest opera I've never seen.

At the top of the list might be Monteverdi's L'Orfeo, one of the monuments of music and operatic history, by perhaps the greatest of 17th century composers. Right next to it is an opera I'll never see, and neither will you, unless the lost manuscript of Heinrich Schütz's only opera, Dafne, comes to light in my lifetime. Yeah, I know, we don't know any of the music, but I've heard, and sung, a lot of Schütz's music, and I'd place a bet on its greatness.

I've never seen Boris Gudonov live, only on Met HD (with a towering cast), and I've seen Janacek's From the House of the Dead only in the DVD of the Chereau production.

But thinking a bit beyond this, and contemplating the list of operas that I have seen, well, a few names are missing, plus I haven't come close to seeing all of Handel's huge oeuvre.

Who's missing? Well, most prominently, the most popular opera composer of the 19th century: Giacomo Meyerbeer. And I've never seen one of Carl Maria von Weber's operas; he is an exceedingly important composer historically. (SF Opera was supposed to do Der Freischütz back in the Rosenberg years - perhaps some day!) Neither have I see anything by Auber, whose La muette de Portici was Chopin's favorite opera.

I've seen only one of Gluck's operas, and none by Cherubini or Cavalli or Marc-Antoine Charpentier. or  Spontini. I've seen nothing by Henze and only two of Birtwistle's operas. Les Troyens is the only Berlioz opera I've ever seen; Benvenuto Cellini sure sounds like a lot of fun. There's a lot of Rossini I've never seen! And, of course, I've seen only one Schreker opera.

And then there's plenty of Russian opera I've never seen. Weirdly, I've managed to see five Rimsky operas, but not his biggest and possibly greatest, The Invisible City of Kitezh. (Hoping to remedy this if it hits the Met.) I haven't seen The Snow Maiden. I haven't seen Mussorgsky's Khovanshchina. No Mathis der Mahler (Hindemith) or Palestrina (Pfitzner), and - perhaps this is the greatest I've never seen - Moses und Aron, by Schoenberg. (Nor have I seen Erwartung!)

Okay, what about you?