SciFi. Why? I'm not Senku. SciFi require a deep and precise grasp of science. We can't narrate SciFi without grounding it in scientific theory and its implications.
The best SciFi I've ever seen is Interstellar—the scientific logic is quite precise. To me, if we truly want to write good SciFi, Interstellar should be the standard just as LOTR is the standard for high fantasy fiction.
If you want to check out a really high-quality Sci-Fi,
The Expanse by
James S.A. Corey (a pseudonym for the collaboration between Daniel Abraham and Ty Franck) is one of the best series out there, and is the personal standard that I have for sci-fi. The TV series was also worth the watch, even if it isn't anywhere near as good as the books.
As for the hardest genre for me to write of those I have actually written in...
Admittedly, it would be Romance.
Out of all the stuff I've written in and dabbled in over the years, Romance is the one genre that I struggle with the most.
Most of the others that I write in just come more naturally to me.
Whether my one romantic-comedy harem series is 'good' or not, well, that's up for debate. But, at least so far, it has been a fun challenge to write.