To begin with, Josei is a manga genre that aims at female adult (or late teens). It is the female counterpart of Seinen, just like the respective words literally means; Josei means Woman, and Seinen means Young Man. Unlike the bubbly Shoujo with its school setting and cliché story, Josei have a more mature story and sometimes more sexual (it is targeted for adults, after all). But just like Shoujo, it mostly revolves around romance, drama, and slice of life.
What makes me fell in love is Josei's more realistic outlook on life, from a perspective of a young woman which of course, quite aligns with the current me. So yeah, I want to share my favorite Josei manga so far. They might not be the best, but I assure you, they are good to read.
Koiiji by Shimura Takako
You may have heard of Shimura Takako from its progressive manga about gender identity and puberty, Hourou Musuko. But for you who aims more at a conventional love story, try Koiiji. Koiiji is a tale of heartbreak, longing, and unrequited love, wrapped nicely in a humble family setting. The art style is cute and the story is relatively tame (as in not too heart-wrenching or complex), so it can be your first leap from Shoujo manga.
Kyou wa Kaisha Yasumimasu by Fujimura Mari
..Well, the description really sums it up. It might come off a bit cliche and the most shoujo-like of all the other titles in this list, but if you're into older female-younger male dynamic and office romance, then this manga is for you.
Sakamichi no Apollon by Kodama Yuki
This series is probably different from the rest of the list, and the reason being 1) It was told from a male's point of view, 2) It took place in Japan on 1966 with prominent school setting, and 3) It heavily emphasizes on Jazz music and friendship. That's why, despite its "Josei" tag, Sakamichi no Apollon is suitable for both female and male readers, be it a sucker for romance like me, or merely a fan of Jazz.
Anata no Koto wa Sorehodo by Ikuemi Ryou
If I was asked who my favorite Josei author is, the answer's gotta be Ikuemi Ryou. She knows well how to toy with the reader's emotion and moral standing regarding relationship, just like she did in this manga. This manga depicts relationship in a more complex scenario since the main theme is affair, which is not everyone's cup of tea.
Natsuyuki Rendezvous by Kawachi Haruka
In the mood for some feel train? Read Natsuyuki Rendezvous. It is a riveting, heartwrenching love story about loving someone and letting go. The manga is also well drawn, I love how the manga draws flowers in detail.
Honey & Clover by Umino Chica
Yet another Josei manga with a male lead and was told from his perspective. Honey & Clover is refreshing, full of relatable and/or lovable characters, and will bring this nostalgic feelings from your college days.
Shouwa Genroku Rakugo Shinjuu by Kumota Haruko
Again, we steered a bit from the run-of-the-mill Josei manga. Shouwa Genroku basically revolves around Rakugo, or the art of Japanese traditional storytelling, with a bit of romance, drama, and tragedy. Compared to the rest, this manga is more elaborated in the 'career' side than the love story, but even non Japanese still can enjoy it (I didn't know "Rakugo" exists until this series, but I'm immersed in the story anyway).
For now, that's all I recommend. If there's anything worthy to add, I will update it or make a new post. Some of the most popular titles like Chihayafuru, Nodame Cantabile, or Kuragehime, for example, don't make it to this list because I haven't read them yet and I prefer to give exposure to the less popular ones.
Well. Hope they can be a good companion in tackling your adulthood and slow days :-D