da moog

F:
Shimokura Vio (Sumaga, Totono) - Easily the worst writer that people pay attention to. Sumaga is so fucking unfunny and terrible that I have nothing to even say about it.
Takehaya (Asuseka, half of Kanishino, Konakana) - Line by line his prose itself might not be that bad but good lord is he an atrocious writer. I read the trial for his new game at ABS2 (v13904) and I couldn't believe how stilted and unnatural the dialogue was. It doesn't help that he's painfully unfunny and has the heroines constantly talk about how great the protag is for no reason. Or that his settings literally exist only to force drama.
Ryuukishi (Fucking Ryuukishi) - Easily the worst writer that people once paid attention to. At least Ryuukishi is funnily bad, though, I guess.

D:
idk lol

C:
Watanabe Ryouichi (Harukuru, Natsukuru) - Prose itself is pretty basic but it gets the job done, funny and has crazy ideas, researches his games a ton. Pretty much the example to follow for people who don't want to make the prose itself super fantastico.
Kinugasa Shougo (Akatsuki no Goei, Reminiscence) - Honestly his prose isn't anything special but he's funny and good at making likeable characters. I've read everything by him, including his shitty LN, so he must be doing something right.
SCA-Ji (Subahibi) - He could reach B tier for me, the potential is there in places, but he'd need to clean up his text a lot and actually pay attention to what he's writing. So many awkward lines in Subahibi especially...
Hiei Murasaki (Baldr Force/Baldr Sky) - Incredible plotting ability and can get pretty 熱い when he wants too but occasionally muddy text. Gets the job done in any case.

~~~arbitrary wall~~~

B:
Shumon Yuu (Itsusora, Asairo) - He has some awkward/strained wording in places and it kind of sticks out and makes things harder to follow than they should be, but he has a great sense of rhythm and knows how to use recurring symbolism. If you can get into his unique style you'll probably fall in love to the extent that you overlook his flaws, I find his comedy pretty funny too. I wish he'd include more physical descriptions in general I suppose. Notable for having such a hard-on for mythology and folklore, which I can appreciate.
Sakurai Hikaru (Steampunk series) - Depending on how much you gel with her style I can see putting her in A rank, I probably would too but yeah. Her style can be a little tiring to read and certainly picks its audience but nobody else really writes anything like her super poetic style in eroge so she definitely has her niche. Also apparently she writes incredibly quickly and can finish the script for a game in a month which is frankly unbelievable.
Fujisaki Ryuuta (Draculius, main for Grisaia, other random shit) - Fujisaki is fucking out of his goddamn mind moreso than any other writer, even Watanabe, and is fucking hilarious in the process. I'd be tempted to put his stuff from Grisaia in A rank but Grisaia itself is a pretty meh story propped up almost entirely by the prose/humor/characters so I can't really give him that high in good faith, I guess you could say his writing lacks depth in that sense. But if you're looking for a hilarious writer who knows how to make his text interesting then Fujisaki should be near the top of your list.
Onikage Youji (Kikaijikake no Eve, Amatsukaze) - Brought down a bit by the other writer who worked on Eve/Amatsukaze and the pretty high amount of random typos/weirdly worded lines/mistakes in both games but I'm willing to overlook most of that due to how overworked he was, especially in Amatsukaze. His stuff has an immense amount of descriptive text for a VN and is written in third person so it comes across as more like a proper novel than the vast majority of eroge, pretty nice and vivid style if you can handle how damn wordy he is. Also has some cool chuu2 and pseudo-wuxia, pretty much the standard of what I'd consider a good eroge writer.
Urobuchi Gen (Madoka) - If you ignore the actual content of his stories he's A tier, but I'll put him here since I wasn't really blown away by anything he wrote except Kikokugai. Really vivid and descriptive writing with a super huge vocabulary, pretty much the best at describing action scenes and giving you a picture of what's happening in your head in general other than Jackson

A:
Masada Takashi (Dies irae) - No one is better at chuu2. His text is pretty much its own "thing" at this point, you'll find more than a few people on 2ch/EGS/wherever talking about how they have a hankering to read his text in particular and that's the way I feel, nobody else really gets close to the particular grandiloquent way he words things. Gotta love his dialogue too, along with all of the crazy research and literary/cultural/mythological/religious references (usually relevant, even) he crams into everything. Also excellent plots and characters that show he really understands storytelling and isn't just following the crowd, he knows exactly what he's doing at all times.
Hino Wataru (Ruitomo, Comyu) - Hesitant to put him here since he's constantly plagued by subwriters but when he's actually contributing to the games he works on he's pretty much top-notch. Akaneko's route in Ruitomo is the only thing that can compete with Romeo at his own game except JQV, if you ask me.
Setoguchi Renya (Kirakira, Swan Song, Carnival) - Well, his games are pretty much just straight up novels with pictures and he's very good at writing novels. I read one of his LNs, Shitai Dorobou, recently, and was pretty damn impressed with it, you would never be able to tell that he was originally an eroge writer... for better or worse, I suppose.

A+:
Tanaka Romeo (C+C etc.) - Looking at prose and content together as a whole there simply isn't anyone better consistently working in eroge.