What I find so great about this work is that it's possible to have completely different preferences within the game and yet love the whole thing just the same. I'd say my top 3 would be:
1) Nancy. In my honest opinion, any top 3 without Nancy is simply wrong. She's ridiculously hot and she wants the mc so bad she actually uses her powers as empress to seduce him, after getting some pretty clear signals that he's into her.
2) Penny. I think this is probably a controversial choice, but to me the combination of her being very comfortable with her own body, and yet somehow slightly restrained sexually is very intriguing. She seems like she needs to unwind a bit, and wouldn't mind some flattery.
3) Alex. I didn't really like the last Alex scene in a way, but I think it makes sense from a story perspective. It seems she tends to use sex almost to create a sense of emotional distance, a "haha, see, I don't care about anything". That's probably not actually true, and I look forward to a scene where she will get emotionally vulnerable, which may or may not involve a fit of passion...