Sex scenes. 
I'm fine with heavy emotional scenes or action scenes or eerie atmospheres but just not sex. It comes out too mechanical and I skip over a lot of stuff lol. Thank god for my betareader! 
Dialogue I find its hard to make it flow naturally. Maybe I don't talk enough, so I'm not sure how it should be done (?).
I talk all the time but dialogue is still hard for me, but maybe that´s because I like dialogue in english better than in german
and english is not my first language
I have a slightly different problem with those, I write them without any trouble but then when it comes to publishing I get too embarrassed and edit them right down 
I kind of struggle with two.
Descriptions: I think I describe enough to paint the picture but I often worry that I'm not doing enough to describe my characters and scenes but I also don't want the descriptions to sound mechanical. Like the descriptions that say "he was 6'2" with a taper cut half-fade haircut and a sleeve of tattoos on his left arm." I just can't tell sometimes if I do enough.
The other is romance. I have characters that in my head, if they were real would be doing the couple thing where they're always half-hugging each other when standing by each other. I can write things like "A grabbed B's favorite X" or "C made D dinner because they were stressed". I can do behavior like that but you can't really write about the tiny touches, the longing stares, etc that IRL couples do around their partner. It's hard.
Thanks for the replies so far. It is interesting to read what other people have trouble with writing or is easy for them.
It´s not like the "hands" answer you get from most people who draw
For me, it's plot. Character dialogue and interactions come so naturally to me, as does imagining them in various scenarios, but pulling those together into a cohesive plot is a struggle.
I find romance to be the most difficult. I usually try to keep it almost entirely out of my writing. I should write some romance short stories to improve myself at it tbh
Romance, don't even start me on kissing/intement scenes. Want something ridiculously difficult, it can be easy for some folk but for me it is so choppy and sometimes it is just ... bleh.
Lolol that's interesting!
Maybe just cover your eyes and press "Post". That way you won't get embarrassed. 
By hardest if you mean requiring the most effort, then -
Comedy. The timing aspect.
Hard Science. The Research and Study aspect.
And almost everything when I write non-fiction.
STEM subjects. Totally not my field of knowledge.
Like I recently got an idea about a machine-driven apocalypse but I had to wonder would my idea for the thing that led to it even be possible? And I have no idea how to even research it because it's so speculative.
I love dialogs and I see myself as a dialog writer more than a settings writer.
I'm not good at settings and describing backgrounds.
I'm just happy that I have a talent artist @nathanKmcwilliams understanding me.
And now I even have a concept artist @Chita helping out with my Bunneh story.
Fight scenes! Can't believe I forgot about that, haha. I've always had trouble with fight scenes. It's hard to come up with anything more interesting than "Character x attacked! Character y dodged!" and so forth.
I'll have to second jobs/mindsets completely away from mine. I'll take weeks and crack my noggin figuring out a believable romance, but I'll get there, the problem is just editing it. But if I have to write someone that's STEM-minded or extremely calculating and have to show that at all times? Goodness. Even my close relatives that work with those areas are still on the more creative side of tinkering and discovering... I can't wrap my head around people that would be entirely uninterested in all that.
I suppose this is cheating but... If I don't like it and I find it too difficult to write... I don't.
There are, of course, exceptions of things that you do have to have in a story such as dialogue and I struggle a bit with description so I work on it. But specific genres? Nope.
If a paycheck depends on it, I'll put every effort into it and do my best hoping that it passes.
I'm not in school so if someone gives me a "prompt" that I don't like I send them that John Travolta hands spread "WTH" gif and blissfully go ahead with my day.
I'm a grown adult. If I don't like or don't want to do something and I don't have to... I don't.
Now, I can understand people saying I should try, and I have, in the past given a few different things a whirl, but, I tend to write what I like and if I'm not into bodice ripping sinking fangs into long white throbbing throats movies, books or tv, so I'm not going to write it.
Unless there's a paycheck involved.
Trust me, no one is going to come to your house and cut your hands off because you refuse to write what you don't want to or find difficult.
Do what you do the best. Pour everything into it. Challenge yourself to grow, of course but don't feel pressured by others that you HAVE to do anything.
I know a lot of people will disagree with what I said, but if you're heart's not in it you won't like what you're doing and when you don't like what you're doing... it shows.
For me it's using situations and language that feels authentic and fun--but isn't hella cliche. Romance cliche's in dialogue are like...all kind of terrible, real cringe, and just so saturated in all our music and movies and books, that trying to find the right words is rough. Like my instinct is to say things like "you are my everything" like everyone else does, but damn that's so cliche at this point, my readers would be able to recite the entire chapter without me writing it. So, I have to step back and say "how can I rephrase it to be something that my readers maybe haven't seen too many times before?" To keep the romance interesting (and mostly interesting for me, because in the end I'm the person I'm writing for--and I'm hella picky about my romance)
Dialogue. Character's way of talking and thinking and how they use their words in certain situation does reflect their personality. To write a dialogue that show's their personality is tough.
Editing. I'm okay with writing, but to edit or polish a sentence to make it better? I'm completely mediocre in that area.