Mozart, Fugue in C Minor for two pianos, K. 426 (1783)

Hideous! Fiendishly ugly! Max Reger--nay, Arnold Schoenberg himself--never wrote anything more brutally unmusical.  I don't think this can be taken as anything but a kind of joke on Mozart's part: a sort of Musikalischer Spass parody of the Art of the Fugue; rather as if Mozart were contemplating the unmusical possibilities of the Bachian counterpoint that he had only recently come to be acquainted with.

P.S.  Just listened to the (probably original) arrangement of this for string orchestra, which is much easier on the ears.  Also, playing for myself, on my little electric piano, the Bach-like fugue (K. 394), which has its deliberate uglinesses, but does not entirely lack charm, shows how this could lead to some of the more wonderful contrapuntal passages of Mozart's late-late manner--I am thinking, of course, of the last movement of the (rather neglected) viola quintet in E Flat, K. 614.