Personally, I think the hardest stuff for me to write about is fiction. I went to school and got a degree in writing and, even through all that time, I still find fiction the hardest thing to write. It's not writing it down that is the hard part, it's getting to a point to where I actually enjoy it myself. If that makes sense. I don't want to write something that I'm going to be bored with later on, I want to write something that I can let sit for a month or so and then come back to edit it and still be entertained.
For me this is tough. I can pump out a review (which is just my opinion) quickly when i get to actually sit down and write, and be satisfied instantly, but for me fiction is rough. Poetry is another thing that takes me forever to get too. I am definitely not the best poet nor do I think that I'm a very good one, but that something else that takes me forever to actually "finish" because even after months of being content with what I have, I still pick at the poem or story and play with it.
The writing process is never complete lol.