Part of the reason is relation to my road to lunatics. At least for top-4. Personal experience with danmaku is a major factor. After all, danmaku is a way of self-expression.
For example, my first Extra clear was in PCB. Music and danmaku are getting surprisingly enjoyable when you are making actual progress. Although I liked Necro-Fantasy even before that. Normal 1cc took quite a few attempts back then. As for initial reason for my determination, I wanted to see Phantasm boss. I succeed. But in the end, Ran became my favorite.
Normal 1cc in MS is kind of turning point, it's when I realised that I'm actually getting good. Shinki itself is impressive. Her danmaku is something special, and so her role and abilities.
Aya is related to Touhou 11 and photo games. Firstly, Reimu+Aya is the best shot-type ever. And I enjoyed their story more than other's. I even got my first Lunatic 1cc in Touhou 11. Besides, I cleared all scenes in 9.5 and 12.5 and for some reason I liked how she does her job. I cleared all scenes with Hatate too, by the way.
Raiko got pretty high because I like Touhou music. That's it.