I'm a fantasy writer at heart. It's probably the genre I'm best at writing, at least in my opinion. However, it's only the second-easiest genre for me to write. If I knew two years ago what genre would flow with the greatest ease now, I'd have been amazed.
A lot of science fiction writers have some form of science degree. I don't even have my high school diploma yet. On top of that, I haven't read nearly as much sci-fi as I'd like to have. But I really enjoy the genre, and it isn't a huge pain to write. My range of subjects is limited without a large amount of research, but I can do enough with the genre with my present knowledge to keep me writing. Zento is my favorite character to write, so when I write sci-fi I typically use him. If an idea can't involve Zento or the Zento universe, it's back to Worldbuilding 101. Luckily, I like worldbuilding.
Horror is a really hard genre for me. Most of my horror stories come from prompts. I read almost zero horror, so it makes sense that I don't really understand how to do horror well. Sometimes I get lucky, sometimes I don't.
I have a lot of fun writing military/war from time to time. It's not very marketable at short length, but the action is fun. M/w is only slightly more difficult to write than fantasy. Character is the hardest to get right. I use more description in my action scenes and I haven't mastered building character while also giving a lot of flesh to my settings simultaneously.
And now, the moment of truth. The genre I have the least difficulty writing is (clean) teen romance. I love how much depth you can put into a story without distracting from the plot at all. You can build characters and setting at high speeds through commenting on visual details and emotions. If you know what you're doing, you can do a lot in each sentence. I don't know that my romance stories are any good (and again, they're clean, so "good" means pretty much the same here as it does for spec fic stories, not what some people would consider "good" for the broader genre), yet they entertain me enough that I shall continue to write them no matter their quality. My first romance story, which I wrote less than two years ago, was really rough, but it had a rawness of emotion to it that really drew me into the genre and coaxed me into continuing to experiment with it.
How about for you? What genres are the easiest for you to write? The hardest? And what genres do you enjoy writing in the most, even if your purpose is not to sell all those stories?