Well, I think it might be either Hermione or Cho. I agree on what you say about how Hermione reads lots of book and how Cho is older than the other three. But I have one more reason on how Hermione is smarter.
Hermione is always top of her year, and that doesnt mean that Cho, who is older than her, is smarter.
No. 1: Hermione
She reads lots of stuff and comes top in everything.
No.2: Ginny
She has lots of pwoerful brothers and can be said to be the best member of DA( just look at her the HP5 movie )!
No.3. Luna
She looks more powerful than Cho...
No. 4: Cho
She seriously needs DA meetings, shes hopeless at magic.