It's not easy to pick favorites in a series where the cast is so brilliantly varied and unique, but I'll do it anyway!
1. Cirno
No, not the fandom nineball-Cirno, the Cirno who went from being a stage 2 boss to becoming the protagonist of her own game and canonically beating MARISA of all people (Sure she says she was going easy on Cirno, but she does freaking admit even if she wasn't she'd have a hard time beating her.) I love development in characters a lot, and I just find it amazing that someone so dumb is actually pretty much on her way to becoming 'The Strongest'. With how she managed to hold her ground to face off with other characters in Touhou 9, to how she successfully managed to fight UTSUHO without melting, to how she defeated Marisa. Top it off with a catchy theme and a lovable personality and you have a pretty charming fairy.
2. Mima/Yumemi
I honestly can't pick between these two, they're both iconic characters from the PC-98 era with brilliant points to them. First of all, everyone knows Mima (she'llbeinthenextnameplsplspls) The fact she's probably had the longest history with both protagonists makes her all the more interesting to me, being Marisa's mentor/possible mother and having a prominent role in the PC-98 games, but there's also a huge amount of mystery surrounding her which just makes me like her more. Top it off with the fact she has some outstanding themes and you have a pretty great character. Now, onto Yumemi. I always like characters that show great strength despite all the odds, and who better an example for that than Yumemi? Before there were gods, vampires and revolutionaries, there was an 18 year old human prodigy. Add the fact she has one of the most badass and iconic themes in the series as well as an AMAZING cape and you have one of my favorite Touhou characters to this day.
3. Mokou
What's not to love about Mokou? SHE'S THE ONLY TOUHOU WITH PANTS! Okay on a serious note, I really love her character, her design reflects her power and her tomboy-personality really damn well, as well as her having a pretty detailed backstory that makes me sympathize and honestly respect her. Not to forget, her theme as well is on a godly level of it's own. Stillcan'tgetpastHonestMan'sDeaththoughdammit.