What do you want to hit the screenplay writers most for?
Goodness, there is so much. As I said, I generally think they did a decent job with the films; however, there are so many things I would have loved to be included, so instead of giving one scene, here's a ranking of the most annoying things the Harry Potter screenplay writers did to the movies.
1) The House-Elf Disregard
In book four, Hermione is obsessed with S.P.E.W, her house-elf deliberation campaign which got entirely cut out in the movies. Admittedly, there is so much happening in the fourth book that they could only screw up the movie; still, this part would have been vital for the later development of the story and how other magical races are being treated by the Wizarding Community.
2) The Veela Disassociation
Fleur Delacour generally got screwed up in the movies and instead of the posh, slightly full of herself but generous and loyal quarter-Veela, we get a girl bordering on wallflower whose appearance is appealing but far from breath-taking. It was a shame they cut out the scene where Bill gets attacked and she steps up for him and the whole her-being-at-the-burrow showdown with Mrs Weasley.
3) The Tonks-Lupin-Omission
In the sixth movie, Lupin and Tonks suddenly are a couple and no one knows when that happened. To worsen it, they aren't even a couple in the book at that point because Tonks only manages to convince Lupin she doesn't care about his werewolf condition after Bill was attacked by Greyback - which is at the end of book 6. I also thought it was insanely sad they didn't include Teddy Lupin in the films, as he represented a strong sign for the circle re-emerging.
4) The Memory Dissatisfaction
One of the crucial parts in the fifth book is when Harry drops into Snape's memory of James Potter torturing him. We do get this scene in the movie, but it is terribly shortened and the gist of the scene is never revealed. There is so little shown from James and Lily anyway that they should have included this memory in the film (instead of panning hundreds of times over Umbridge's cat pictures - we get it, people who have cats are evil).
5) The Quidditch World Cup Excision
Again, the fourth book was packed with action, but the Quidditch World Cup would have been amazing to watch and it is sad they didn't include it in the movies, as it would have added positive action to all the dark stuff happening later on.
6) The Family Extinction
In the first movie Harry finds the Mirror of Erised and sees his parents. It is a beautiful scene which is also why I included it as the last point in this list as, per se, it was a great scene; however, in the book Harry sees his whole family - grandparents, uncles and aunts, etc. - and it would have been great to not only see Lily and James but his whole family.
Of course, there is much more that should have been included and I hope dearly that one day, they will do a TV show remake of Harry Potter (GoT style) where EVERYTHING will be included.