The worst, in my opinion, is The Murder of Roger Ackroyd. I felt this was a travesty of her book, considered by many to be her masterpiece. I realise that it is a difficult one to adapt as much would be given away if we were really to see everything through the murderer's eyes. However, that shoot-out at the end was more like a scene from a Batman movie.
Another poor adaptation was Dumb Witness. To introduce motor racing seems irrelevant, and why was Emily Arundell alive? One of the clever plot lines was that Emily's letter was delayed, and by the time Poirot received it, she was already dead.