The creative spirit of a composer is demonstrated through the impossible task to exactly replicate what he/she has heard in his/her environment. However, what happens is that he/she reinvents what is heard ad infinitum.
In this vein, the variation form has become a driving force of the composer's creative impulse. Two main different forms of elaborating the original material have become the source of achieving even the most complex and lenghty variations: by ornamentation, amplificatioin or deviation of the melody or, based on the bass line, variations on the harmonic progression or rhythmic changes.
While the Variations for one instrument or chamber groups are more common and very well-established (e.g. Bach's Goldberg, Beethoven's Diabelli), the Variations for Orchestra are not that many, while rather few are quite well-known. However, they are more complex and intriguing (particularly as for the orchestration), quite individual, often lengthy enough and sometimes grandiose and even profound.
Some of them are very well-established complete works (e.g. Elgar's Enigma Variations), but quite a few appear in a movement of a Concerto (e.g. the last movement of Mozart's Piano Concerto no.17, in G, K.453) or in a Symphony (e.g. the last movement of Beethoven's Third Symphony).
So, how many and which ones could be identified as the most significant Orchestral Variations, in any form (complete work, Symphony or Concerto's Movement)?