Basically, what it says. Which of the official Touhou manga series do you like the most - Silent Sinner in Blue, Inaba of the Moon and Inaba of the Earth, Eastern and Little Nature Deity/Strange and Bright Nature Deity/Oriental Sacred Place, Wild and Horned Hermit or Forbidden Scrollery?
Myself, I find I started really liking FS lately. I really like the drawing style and the story is engaging, plus Akyuu is shown a lot, she really needs some attention. I actually didn't like it at first since Kosuzu looked like she's too likely to make problems for no good reason, but she's grown responsible by now and does whatever it takes when there's a danger, so I've grown to like her.
Honorable mention goes to Three Fairies mangas. I really liked the slice-of-life format. However, the artist's style, while working fine for fairies themselves, looks a bit jarring with other characters (especially Aya, whom I've always seen as extremely lean). Plus there are things like Cirno's eyebrows and artist's fondness for a certain exact standing pose. But these things are quite minor, the series is pretty enjoyable.
(A poll would be nice here, and I think I could add one when I had Birthday powers, but since I no longer do... welp :V)