If I were to start reading the Touhou manga, which one would be best to start with.
Depends on what you're looking for I guess, most of the manga stories are somewhat self-contained.
The series dealing with the Lunar People, Bougetsushou, that I would consider fairly serious, or at least more serious than the games take story and worldbuilding. But it has some interesting tidbits about the lore around Gensokyo, particularly about the Eientei faction (obviously) and Yukari. The first two books (Silent Sinner in Blue and Cage in Lunatic Runagate) are a complete story, and the third book is a collection of comedic 4koma detailing the aftermath of the first two books.
There's Sangetsusei (Sangassei?) about the Three Fairies of Light which is very light hearted and adorable and will make for an enjoyable read. There isn't really an overarching story, it's very episodic and chapters usually do not follow on from each other in terms of story. Has light amounts of worldbuilding and lore.
I haven't finished Forbidden Scrollery (Suzunaan) or Wild and Horned Hermit (Ibarakasen) so I can only offer partial knowledge about them. Forbidden Scrollery has a darker edge to it than the other manga, but there are moments of levity. It generally explores the going-ons of Gensokyo from a non-powered human perspective; though the main character has a special power, it's not one that makes them good at fighting. The protagonist is a book shop clerk living in the Human Village and the deuteragonist is Hieda no Akyuu, who is a prominent figure in the Village whose role is to record history through their eyes (the clan's special power is being able to recall memories from previous lives at the cost of a short lifespan). Reimu and Marisa are major characters but not the protagonists. From what I hear from other people, there is a complete narrative arc over this series so reading from the start is absolutely advised (although the same can be said of other series I guess).
The other one is Wild and Horned Hermit and this one does feature Reimu as the protagonist, along with new character Ibaraki Kasen, who is a hermit disgruntled over how un-miko-like Reimu is and decides to guide her to becoming more "pious", to varying levels of success and hilarity. I don't think this one has an overarching narrative like Bougetsushou and Forbidden Scrollery. In tone it is fairly light, and nothing really
bad happens, though some exposition can be a bit dire. WaHH contains interactions between a lot of the major cast of players previously established in prior games, and also gives a view into the day-by-day life of Reimu and her attempts at avoiding super poverty.
I would also like to give an honourable mention to the Symposium of Post-mysticism artbook, which contains a story in the form of meeting minutes between Byakuren Hijiri, Kanako Yasaka and Toyosatomimi no Miko with Marisa acting as moderator to discuss various circumstances in Gensokyo. As a fan of worldbuilding this book was super interesting. You can't sleep on the other artbooks either which I would also recommend reading through.
So I would say pick whichever one you're in the mood for first and then move to the overs as you go.
