Boys Over Flowers was such a painful read for me. It is... just... horrible.

I find it funny that almost all listed in here are just mainstream mangas. Mainstream doesn't equate to bad you know. But some do because I'll list mainstream too, just poorly written.

Special A, Kaichoi Wa and Ouran high are all enjoyable for me. Just entertainment and rereadable at that. Books were in you don't need to overthink.

Preferences plays a lot.

I guess I'll list some:

Hirunaka no Ryuusei - aggravating. The sudden turn of the plot kills me to death, I would never reread this manga.

Hibi Chouchou - cute at first....gets illogical quickly. I mean a lot of mangas are illogical at some point but not this much. It's laughable if you debunk this manga. A copy paste of Kimi ni Todoke with a twist...annoying at that.

Mairunovich - .........see for yourselves. Not exactly a bad story but the execution.

Parfait Tic - the most FICKLE girl ever! Yes the main character has problems yes, yup, but oh gaddd... I hated her character so much. There are types of hatable character yet you'd understand and sympathize like Heroine Shikkaku's Hatori, but Fuuko, geez.

Ato nimo Saki nimo kimi dake - Big FACEPALM.

And a lot more. Some are already bad to be even popular. Vice Versa, popular but bad.
Those manga that has pointless love triangle are even worser.

Everything OP listed, along with Bride of the Water God, Special A and Hadashi de Bara o Fume.
I was about to list Midnight Secretary, but I believe it's a Josei... oh well. I hate it enough to mention it anyways.

I try to finish most of the stuff I pick... although this will was more powerful some years ago than now and I wasn't very picky, which made me read a lot of crap (along with the good stuff of course). But it wasn't hateful, remarkable crap.

Anyway, I used to love Hana to Akuma six years ago. But then, 14 chapters prior to end it, I got pissed by Hana and her 8 years old kid looks and actions (yeah it took me 34 chapters to drop it because I really wanted to finish reading that). I also didn't go on with Mei-chan no Shitsuji.

Aikawa Saki's and Tanemura Arina's works piss me off as well... Shinjo Mayu is avoided and Hot Gimmick is a given. Also, Wagamama na Butler... just no.

Kaichou wa maid sama, Switch girl, Cheonsaeng Yeonbun, Kodomo no Omocha, Nisekoi Doumei........and the one which tops my list.......Nanohana no Kare(Nanoka no Kare). It's better to jump from a building and die, rather than touching this manga. This is the worst of all.

The one I disliked the most was Happy Marriage. The love relationship was REALLY toxic and kinda abusive but it was portrayed as an idyllic

Oh, dear, there are differences in what shoujo and josei are.

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...oh boooy
Heroine Shikkaku
Peach Girl
SAKISAKA Io's works :\ (They are just so bad in my opinion ugh)
Orange Marmalade

Pretty much anything with a whiny, weak, meek, push over female lead and playboy, hotshit, stuck up male lead lol

Ohhh, I can think of quite a few for this.

Hadashi de Bara o Fume - At first, I remember really loving this. It was cliche, sure, but I thought it was pretty interesting. All of a sudden, the rival who fell for the female lead went crazy and it just went more and more down hill. The writing just went down the toilet.

Kyou, Koi wo Hajimemasu. - It's been years since I read this but this might be the worst thing I've ever read. I was interested at first (by that, I mean through the concept/summary) and sat through many, many chapters. Everything was just terrible, though. If I remember it correctly, my urge to drop the series grew more and more because the female lead said she wanted to save her virginity for after marriage ... and the first thing the male lead did was stop touching (just kissing and hugging is all they even did, I think) her because he wanted sex. That just leaves a really bad message about dating for younger readers. I basically couldn't stand either of the main characters. (Oh, and even the protagonist's sister was a little brat.)

Aside from that, practically anything by Minami Kanan (author of the second series I mentioned) is usually bad along with sending the wrong message to younger readers. I believe she has a more realistic one-shot story about rape that I actually enjoyed though.

When it comes to bad manga, the same can be applied to most shoujo smut stories as well.
They generally follow the pattern of the guy forcing himself on the girl and she just falls in love with him because of his amazing personality.

* EDIT
Oh, and I forgot Heroine Shikkaku. The art was lovely but if you want a cast of terrible characters with selfish and childish personalities, that's your story right there.

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Quack, quack.

I read a lot of psychological or shoujo manga for some reason. Preferably not mixed together.

Vampire Knight was SOOOOOOOOO BLEHHHH.
Boring! And it was weird! All the characters felt... Lifeless... And not in the vampire kind of way.

Also Golden Time was aggravating and boring. I couldn't get myself to like any of the characters. Ba to was too weak and Koko was just annoying. It didn't make me cry at all or feel anything really.

To be honest I don't really like most mainstream shoujo and I don't read smut, because I am not mature enough in mind for that. I don't like Kaichou wa Maid-sama, Vampire Knight and I am not so fan of most manhwa and Ouran either.