Reisen really rose up in my ranks due to sheer character development. She starts off as an elitist asshole and changes so much that not only does her very race/essence change, but her personality and will changes enough that she's willing to protect Gensokyo in spite of her Master saying they shouldn't get involved, simply because it's the right thing to do. This from someone who once went AWOL from her home nation and knew little of heroism or loyalty!
It's great. She ties for me with Yuuka, though, because I just plain like Yuuka.