Yeah, I suppose they do take from the rythym genre like Sekiro does. Perhaps they are the one genre but I can't even tell what high level players are doing in those games to know for sure. But that being said, Sim racing also has Manuel gearing which is wildly difficult to master while you're racing for millisecond times on tracks. 

Action would be the most difficult for me. Is that counted under "Soulsbourne Action"?

Easiest would be RPGs, strategy games, and sandbox simulators - Dwarf Fortress has the reputation of being one of the most difficult games to learn, but it's actually very simple if you start by isolating the basic survival mechanics and work out. You can probably master the game in 8 hours. Most strategy and RPGs are even simpler than that. RPGs, IMO, are the easiest genre - even action RPGs like Witcher 3 are quite easy on the highest difficulty levels (Witcher 3 Death March is only slightly more difficult than the basic difficulty), it's just a matter of character setup and understanding the mechanics. I also love these games :)

Next to easiest
Moderate: Platformer, fighters, racers. But, again, these depend on the game. Some will be easy to master in about 2-8 hours, but many take around 20 hours to master.

Moderate: Puzzle, I find these take quite a while to master, and even then you can improve an even more. 30-50 hours to master?

Hard: Action games, particularly older ones from the 80s and 90s. Some of these are insane! Ghosts and Goblins, FUCK that game :)
It's been 30+ fucking years and I still am nowhere near mastering it!
Or how about Fantastic 4 on Playstation?
Of course, there are many many exceptions, Turtles in Time was quite easy, for example.

Hmm, this is more knowledge based than skill based, you can easily learn. I doubt it would take 10,000 hours of researching and playing these games to become a master at them. I have 2500 hours in FF14, I had it mastered in the first 500 hours. 

Personally platformers, rhythm and fighting games are the hardest for me.
I haven't really played souls-likes, but from general action I don't think they'd be the hardest genre for me.
In FPS, tactical shooters are more a problem, something I haven't played much.

Grand Strategy games.

You give someone Europa Universalis or Hearts of Iron without dedicating the next 100 hours to explaining how the games work, they’re immediately giving up lol

You definitely need skill for any game, and a skill can be anything really. My skills are not RPG games and Statistical games. Heck, I have played a War game on mobile for the past 6 years since leaving vgchartz. No idea how it works, other than some marches kill others. Still play it, slowly learning. My other skills, which are few have made either where I am still successful, but would be so much more if I could just work on the skills I struggle with.

You're missing RTS. Probably that since it has the most moving pieces. Otherwise I'd say bullet hell and platformers, there are certainly some games in those genres that makes soulsborne look like a cakewalk (though ofc the average platformer is very accessible).

True. Didn't even consider them. 

Updated poll. Anything else I'm missing?

RTS was my first thought.

The skills needed to be good at Real time strategy is in its own league. You need reflexes like action games, individual and multiunit control, great game plan strategy - to be changed with new info, mouse precision handling like FPS as well as sick multitasking ability. To be competitive in RTS demands a lot from the mind.