Definitely Luna for me. I also feel that Hermiones tits look fakish and too big for her character. As someone who has read the books, as far as the characterization goes, Hermione definitely seems the most out of character to me, both in personality and body.
As someone else has already said though, we haven't actually seen her breasts, since these might just be the way they look when pushed up a bra. I'm also hoping the artist is gonna add a full bush to her, since that seems so like Hermione. But I get that it's not everyones thing.
As for Luna, I really like what they did to her. She is pretty much just like in the books, except more horny. Perfect for a nsfw game.
The other two girls aren't really described in the books, so they were pretty much a clean slate.