Which art form is the most difficult, in your opinion, and why?

As a career professional classical musician, I will of course vote for music. We need to practice every single day of our lives to maintain the extremely high degree of technique that this requires. When I worked in popular music, or accompanying of casual singers, choral groups, etc. - i could easily skip a day ore more, and I would be fine - had to practice the few knotty parts, that's all. My late Dad was a calligrapher and commercial artist - and I know for a fact that you do NOT need to pick up those quill, brushes, or pens every single day. I know that fine dancers (ballet, etc.) take class every day - it is necessary to keep those muscles at top function, the same as we musicians must. The old joke among us is that if you take ONE day off from practice - you can tell. Two days - your colleagues can tell. a week? The whole world can tell.

Due to holiday cooking and baking prep, I will tell you that yes - I missed a couple of days this week. I have to play in public this Saturday. You had better believe that I will be back running my regimen within ten minutes from now (we all have things that will get us back on track FAST if we have sinned for a day or two) - just sat down to check messages while I down that second cup of coffee. Brush my teeth - assemble the flute.

They're all difficult in their own way, and according to the person attempting them.

They may come easily to some, whilst others might find any or all of them extremely difficult.

I can't say, I haven't tried all of them.