>>49963848
I didn't want to answer this yesterday since I was drunk, but, here goes:
I actually disliked Sanae at first. I started with the games and was hopping between 6-10 practicing my danmaku skills, and I couldn't get past Sanae because I was a shitter. All my favorites back then were based on looks and Sanae didn't really strike me as an interesting design. I didn't understand why people would like her.
Then I decided to read the omake, I played as her in Touhou 12, I progressively started getting more and more interested in Sanae as a character until she finally usurped the #1 spot and became my favorite.
So that's the how, here's the why:
I enjoy that she's a nerd and that she's very in your face about it. She constantly makes references to games she likes and is very open about being an /m/tard. She's optimistic, positive, contagiously so. Even then she can be a prick and overly competitive and bold. She likes gloating, she's overly confident in herself despite not really being all that powerful, she's willing to tell the goddess of Hell herself that her t-shirt looks like shit and get away with it. She's genuinely a hard worker, someone who not only works hard for herself and her shrine, but also cleans up after her friends. Despite all her competitiveness, she likes helping people. She comes off as a total tone-deaf dork that doesn't realize that people don't really care about the things that interest HER, but is either unaware or doesn't care at all, she simply speaks her mind and ropes in whoever is nearby to listen to her ramble about her interests. She's easily excitable. I like how connected she is to a real life shrine and family. About the only thing I dislike about her, and many other characters, is that ZUN refuses to give her any sort of backstory that's not the most barebones shit ever.
Finally, I feel as if I'm forever stuck with having her as my favorite. Other characters may periodically grip me and be more interesting to me, but I feel as if I can't just decide to not like Sanae anymore. It's a done deal.