Very difficult too say. Any instrument is difficult. To me, I play a lot instruments. I think flute is the easiest but my flute students never agree. I think recorder is the hardest because it is very difficult to control the intonation and the fingering but a lot of people don't agree. My flute teacher always say, " nothing is easy even one single long note".
Actually it is very difficult to defenite which is the most difficult instrument. Some may say Piano is the most difficult because it needs two hands. And some may say violin is the most difficult because its difficult to make a quality tone. And some may even say the triangle (percussion) is the most difficult because players need a long time to find out the best and highest quality tone.
If you are interested with learning instument, just depends on your hobbies, fingering, and your feeling.