What is the most difficult instrument in the world?

The most difficult instrument to master is piano. There are plenty of musicians out there who play all different types of horns, woodwinds, percussion, strings, etc. etc. as well as vocalists who have tried piano who will tell you piano is much more difficult to master. Piano requires a higher level of brain development and more vast, intense and complex range of musical comprehension than any other instrument. That's piano is theoretically inclusive of every other instrument, so therefore there's a lot more to mastering piano than any other instrument.