Oh, I actually posted something like this before.
To be brief, my favorites would be:
5. Koishi,
for her unique power. I like the idea that can make herself like a distant dream on a whim. Also, because she's pretty unusual, saying she "doesn't remember anything" and that she doesn't even know if she's deliberately controlling her body.
4. Alice,
who I would personally consider the most suitable main character if she was playable more often.
3. Kaguya.
Pretty much the character who got me into the series. I love her wide variety of spellcards and the fact that she has a super long dress. And long black hair, which I have a thing for.
2/1. TIE: Yuyuko and Byakuren
because both are pretty dang unique.
If I must say it bluntly, both have their really charming sides, and both are really dangerous when they need to be, and I like that. I also like how there's something like a sword of a backstory in their candy shell of appearance and charisma, and their role-modeling to me - Byakuren's impartiality and will to unite humans and youkai, and Yuyuko's behavior that switches between lovable princess and witty wisp to her advantage.
On the note of Byakuren, I also like her for being 100% definable as an actual "heroine" in the series, which Touhou was missing for a good while.
This coming from someone who knows the series well but doesn't pay attention to the novels.
I actually like a lot more than these. These are just my top 5.
