If you were only gonna watch one Harry Potter film it would definitely be Prisoner of Azkaban. It was shot with real style and class by Alfonso Cuaron and, makes best use of the adult actors, David Thewlis, Gary Oldman and Alan Rickman shining here. Brilliant effects and location work too. I would watch it any day above Goblet of Fire and Order of the Phoenix, Order being particularly disappointing IMO when the book was insanely good!
E.T. is ace too! It's one of Spielberg's few perfect films along with Jaws, Close Encounters, Raiders, maybe Schindler's List and Jurassic Park (that's pushing it, I know...).