Would you be able to expand on the question, what exactly do you mean?
Is the question what art form in toribash is the most complex?
Or what art form in general is the hardest to master?
Or what art pieces show off the hard skills that were required to make it?
Don't dm me pictures of bowls that you find attractive.
If I'm understanding the question, maybe realism is the hardest since it take a lot of time,
Object would be something with a lot of detail,
Basically the more detail something needs the harder it can get.