What is the most undeservedly obscure piece by Mozart?

My vote would go to the incidental music Mozart wrote for 'Thamos of Egypt'. It contains some first-rate Mozart from the time of the opera 'Idomeneo', yet is almost never heard.

I would agree with the Missa Solemnis being obscure in America. In fact, I think it's obscure all over the world. A lot of people seem to prefer the Mass in C minor, even though he didn't complete it, not to mention the Requiem, which he didn't complete either.

There's enough Mozartian obscurity to go round for everyone, though each of us will understand different things by 'obscure' depending on experience & our exposure to the repertoire. Here's a few, hopefully, to suit everyone's requirements at least once..:

Though known to scholarship as 'the greatest operatic torso of the 18th century', even as recently as my student days in the 1970s, the Singspiel "Zaïde" K344 languished forlornly in complete neglect until a number of high-rent singers resurrected a gem of an aria from it, which eventually led to the performance in full of what material of this masterpiece is left to us, still a rare occurrence indeed. This is the aria that started that ball a-rolling:

"Zaïde" K344, Aria "Ruhe sanft, mein holdes Leben"

http://www.youtube.com/watch?v=mdRnUU5Hqck&feature...

Mozart himself was resigned already in 1783 to the less than stellar career of his sole excursion in the realm of (straight) theatre and incidental music, and this is his only essay in the form: "Thamos, Kö*** in Ägypten" K345 (1773-1779) to the heroic drama of the same name by Tobias, Freiherr von Gebler (1726-1786). Mozart himself thought very highly indeed of his music for "Thamos", rightly so too, and little could he know how extraordinarily prophetic his closing words about it would become, writing to his father, February 15th, 1783:

"I am truly sorry that I shall not be able to make use of the music for Thamos. This play has been discarded here because it was not liked, and will not be performed again. It ought to be performed, just for the sake of the music."

Here's a "Thamos" choral scena (VI) "Gottheit über all mächtig" in a wonderfully characterful performance by Barocchisti (who have actually put all of Thamos online @ YouTube):

http://www.youtube.com/watch?v=-pGyeKe7zh8

Being married to a singer (of sorts), it's hardly a surprise that Mozart should have been struck by novel combinations for a concert aria, but it was actually for Nancy Storace (his first Susanna) that the Concert aria "Ch'io mi scordi di te" K505 (1786) was created -- with a difference as will become clear shortly -- and even now, though more often aired (still only rarely in comparison with much else), it is as rare as hen's teeth to find a performance quite so high powered and simply superb, incomparable even, as on this occasion in Vienna in 2006:

Scena è Rondo "Ch'io mi scordi di te / Non temer, amato bene", K505

http://www.youtube.com/watch?v=--vA7l7cFgo&feature...

(Equally often not realised is that the subject matter for this scena is taken from "Idomeneo" (K366) which, of course, Mozart set in full in 1780-1781)

Finally, for the epicures of obscure, there is his pantomime music "Pantalone und Columbine" K446 which is even sufficiently off the beaten track to defeat YouTube for the present. Witty and very highly wrought middle-period Mozart, it is, I believe, only currently available on CD as part of the Mozart Complete Edition, Vol XVII - 5:21-28, which for obvious copyright reasons I can't put up online. Seek it out if you can!

All the best,

I don't hear Mozart's "A Musical Joke, K. 522 (4th mvt.)" very often, probably because few people get it.

It depends of your definition of obscure in music. But I think I know what you mean.. I suggest :

The Requiem.

I don't think there is such a piece. Very little by Mozart is obscure.