In 1819 Anton Diabelli, a composer and prominent music publisher, sent a copy of a waltz theme he had just written to 50 composers. He requested that each of them write one variation on the theme for a composite set, intending to publish this set and to use the profits from sales to benefit orphans and widows of the Napoleonic Wars. The composite set was published, but without a contribution from Beethoven. Nevertheless, the theme must have eventually grabbed his attention, because by 1820, in a letter to Peter Joseph Simrock, of the Bonn publishing house, Beethoven refers to his work on the variations as, "grand variations on a German waltz." The set of 33 variations known as the Diabelli Variations were finally published in 1823.
Martin Cooper, in his book entitled, Beethoven/The Last Decade, states, "Diabelli's waltz revealed an unexpected number of characteristics necessary in a variation theme---a strong if primitive harmonic structure, salient rhythmic traits and a melodic nullity that was itself a kind of virtue." Beethoven's limitless talents took flight with this theme and when he was done the world was 33 variations richer. Cooper goes on to characterize this opus as, "an epitome or microcosm of his musical world. The variety of treatment is almost without parallel, so that the work represents a book of advanced studies in Beethoven's manner of expression and his use of the keyboard, as well as a monumental work in its own right." Earlier, Hans von Bulow, a 19th century piano virtuoso and student of Liszt, referred to the variations as, "a microcosm of Beethoven's art."
My favorite of the variations are the Fughetta (Var 24), the funereal slow March (Var 14), and the Fugue and final Menuetto (Vars 32-33).