“The King Of Instruments” is a title normally accorded to the organ. However, I know that there are very capable organists who are Quora contributors who will doubtless tell you more as to why this statement has authority.
I am not an organist, so I will justify my instrument the piano, according to your question: Franz Liszt believed that it was possible to transcribe ANY music for the piano, and his own transcriptions of other composers’ works, bear out that theory (although, one would need a 1/4 tone piano to play certain 20th/21st century microtonal works).
My [music] theory teacher during my teen years was an FRCO organist (highest organ diploma in UK), a qualification that he gained at 19; yet he described the piano as a superior instrument because it has a SUSTAINING pedal. This makes it possible ultimately to sound much more complex harmonies than are possible on a sustaining instrument such as the organ; for the notes cease as soon as your hands and feet are removed from the keys and pedals.
A strength of the organ is that its notes sustain for as long as you depress them, while the piano is an instrument of decay; also you have an extra part in that the feet can play melodies. However, one of its most famous weaknesses is that it is not “touch-sensitive”…that is to say, the strength of key depression makes no difference to the volume. A gradual crescendo is impossible on an organ; rather it tends to come in “spurts” as extra stops are called upon to increase volume (the “swell” pedal has a limited capacity in this sense).
Now, the organ has resources of colour that the piano does not, nevertheless there are notable musicians (I assume non-organists!) who claim that organ tone becomes wearisome after a prolonged recital. However, an FRCO organist friend of mine says that BAD REGISTRATION is a major factor in this impression. As an organist, he is listening out for timbres that pass me by, if I’m honest.
One thing that puts all keyboards at a disadvantage from stringed instruments is that the pitches are preset, and so it is impossible to “go in between” the notes; moreover strings, brass and winds can “do” a lot more with a note in terms of expression while it is being sustained. With the piano, you can do nothing more with it, and the organ can only vary the sound through stops as a limited form of expression.
If the piano, FOR ME, is the King of instruments, it is because it is such a useful compositional tool, and versatile in application. However, I consider it vital to study another instrument as well (which could be the human voice), because as a MELODIC instrument, it is inevitably less expressive than many non-keyboard instruments. As a pianist and piano teacher, I am constantly trying to get pupils to SING certain parts of the music, because through the voice that expressive potential can be better understood. Then the task is to decide how best to realize it on an instrument whose expressive limitations I have already outlined.