I have the theory that Chachamaru is by far the most popular heroine (especially in JP) because the readers subconsciously understand that she's the only life-affirming character in the whole game. You would think it's weird to like this borderline side-character the most, who barely has a route herself, but it makes complete sense when you consider her thematic relevance. Though I HIGHLY doubt anyone actually understands why she's the best one beyond subconsciously. Regardless, she's best girl, because she's the most human of them.