Rupert is definitely the most natural actor out of the three, but I think Dan is pretty good too, has been since Prisoner of Azkaban. Emma seems to have gone more overblown in her performance since Prisoner. Maybe she wants to emphasise the difference between herself and Hermione, but I think she was better in Prisoner when it seemed less like she was putting it on.
They probably should have given Bonnie Wright more to do in past films so she could come into her own as Ginny in Half-Blood Prince. As it goes she's had about three lines in 4 films!