Ron and Harry are her true best friends, obviously. But I suppose she's closest to Ginny after that. She doesn't seem close to any of the girls in her own year.

Well if we exclude Ron and Harry (yeah they are the ultimate trio BFFs but since Ron and Harry are supposed to be 'the closest' in terms of friendship from the three) i think Ginny is the other person that can be called Hermione's best friend . Hermione was never shown to interact that much with the girls from her year nor anyone else beside Harry and Ron , but in OoTP she starts to share a room with Ginny and that happens a lot of times so maybe they grew to be something like best girl friends at some point (maybe even alongside Luna , like a girl trio)

Also, Ginny knew that Hermione had kissed Victor, so maybe Ginny was the one she went to for girltalk.