Was going to go with Kimi e okuru, sora no hana, but I honestly liked all the heroines in that game a lot. Might be considered cheating.
Hmm, well then, let's go with Hakoniwa Logic, which puts us with Koko.
Pros: She is a huge M, and would probably be onboard with literally anything in the bedroom. She is also really devoted to the person she likes, which is a pretty great quality, and she is quite a caring person in general.
Cons: Her personality is cute in tiny doses, but 90% of the time it makes me want to scream and run away. Also, her tendency to tie herself up half-naked in the classroom when everyone else has left isn't exactly the best quirk in the world. Oh, and the way she spaces out in her weird fantasies all the time is quite annoying as well.