I have two nominees for this. I've mentioned this before, but gets really bogged down with the harem aspects after several books. The author feels compelled to let every single person in the harem comment on every single thing and also let the reader know that every single person in the harem is totally fine with their arrangement... repeatedly. It starts to read like a bad fanfiction and the author needs to get a thesaurus so they stop using the adverb "lovingly" so many times.

This is just a random page from book 7... the book where I was like "I can't take this any more" and gave up on the series:

Now, I at least made it to book 7 of that series, so it wasn't all bad, especially not at first before the harem is established (even though there's definitely some relatively explicit chapters that I could do without). But the book series where I swear the author just didn't give any shit about his writing or his characters was . It's just... a mess... like the author had no idea what he was doing and just wrote whatever.

Look at this garbage. There's punctuation errors, numerous cliches, etc. and it's all terrible difficult to follow. This isn't LitRPG at all and yet there's stuff about power points and damage values so it's all meaningless. It's like if you were watching a Samurai movie and the guy slashes an enemy and suddenly you see "Raiden slashed for 30 points of damage". This is also the novel series where I found so many major plot inconsistencies that I started called the MC "Amnesia Protag" because he'd just forget he did things in the previous book or even in the previous chapter. The author also seemed to like padding the book with lots of needless explanation. Like, they were going to ride a dragon. A normal way would be "we made a harness and then used wind magic to ensure we were comfortable and didn't fall off." This guy would spend PAGES explaining how he was fashioning a harness out of leather from a boar he had from a fight three winters ago and using rope made from vines and then explain wind dynamics that would need to be "overcome" for the wind magic to work and then five pages later they'd FINALLY get a move on. Oh, and this is also the book series where the random elf stranger has sex with the MC because she doesn't believe that he's not grossed out by her.

Most light novelists are terrible at writing women.

Kumo Kagyu (Goblin Slayer) can't write a single scene with a woman in it where he doesn't make a comment about their body.

High Priestess

She placed a hand on her not-yet-ample chest.

High Elf Archer

Dwarf Shaman found that his stubby fingers were too slow to stop High Elf Archer, who chuckled triumphantly and puffed out her little chest.

Witch

"See, you…” Witch lifted her luscious body from her chair.

Cow Girl

Thank you so much! I’ll treasure it!” Cow Girl said, clasping the shell to her ample bosom and grinning.

Sword Maiden

Her sheer, white garments gleefully exposed the voluptuous lines of her body.

I stuck with it because the side plots were interesting, but I dropped it once it become clear that the main story wasn't going anywhere.

I'm currently reading <Flung into a New World? Time to Lift the 200-Year Curse!> and it has the worst writing style I've ever had misfortune to stumble across. around 90% is written in inner monologue. Let me give you an example of how a typical paragraph would look like:

Well, I have to write a MidWeek post but I'm drawing blanks. What? I've switched to an every-other-week schedule? Yes, that's true but I hope people don't think that I'm lazy. What? You won't? That's nice but I just thought of a question and throw in a bonus post out of schedule to keep you all on your toes. Yes, that's really mean of me. Hey, why are you all leaving. No stop, you're mean now. Well, here is poor me all alone talking into the void again. Oh woe is me. What's that? There is still someone here. Well, welcome. Hope you have great time. What? You don't like my bonus Midweek post? Too bad, I won't remove it! Serves you right!

This is around 85-90% of the entire book. And If you think I was exaggerating:

It's the entire thing in this manner. This is not a cherry picked page but just the random page I was currently on. It's insane.