Ginnevra Weasley of course, even if she is a bit of a playgirl in the book having a lot of boyfriends she would never rat Harry out to the enemy, did you even notice she liked Harry from the Harry Potter and Philosopher's Stone movie? By saying "Good Luck" when Harry was about to go through platform 9 3/4...
Cho Chang is so cruel, in the book she had a date with Harry in Hogsmeade, she was already Harry's girlfriend, in the tea shop they were in (or was it a cafe, sorry I can't remember the name)she was crying about Cedric's death, she kept crying over it and Harry got mad about it and left her, but in the movie, this wasn't how their relationship ended, it was that Cho told Professor Dolores Umbridge about the Dumbledore's Army, so Cho is a bit cruel...