There has always been a long-running debate as to which is the more difficult instrument to play; the piano or the pipe organ. Essentially, there are two fundamental differences in terms of difficulty: The piano is touch-responsive; i.e. expression is possible by tempering the velocity with which a key is struck, and on the other hand, the pipe organ demands the capability of playing with both hands AND the feet!
To clarify, there is no 'expression' possible on an organ key -- the pipe is either on or off -- it's either being sounded or it's not. On the piano however, you can play a note softly or loudly just based upon how hard you strike the key. Piano pedals also allow for adjustment of that expression.
Having said that, a pianist is not confronted with an array of stops (switches which turn different combinations/ranks of pipes on or off) and some organs have up to seven keyboards (including the pedals) so the argument lingers. However, for the listener, the pipe organ is indeed really the 'King of Instruments'. It has a huge range of pitch - from pedal pipes that can be up to 64 feet high which you don't actually hear but 'feel'; to tiny 'pan pipe' ranks that can whistle at frequencies higher than most human ears can hear.
Bach is probably the most famous of all organists and organ composers, and definitely the most prolific. His works span hugely magnificent choral works to delightfully soft 'carillon' organ melodies - each genre showing off the jaw-dropping capability of a large pipe organ.
Another awe-inspiring organ virtuoso piece is the Toccata from the 5th Symphony by Charles Marie Widor, with it's mathematical genius of composition; it's ascension in left- and right-hand arpeggios, and the thunderous climax with 'all stops out on G major' at the final chords. And then the incredible echo as the music resonates around the venue after the pipes have stopped sounding. We are fortunate enough to have access to recordings of Widor playing this himself - although he was of older years at the time of the recording and didn't quite play it at the tempo we're used to hearing it at. But what a genius.