For many, many years, it used to be "The Wizard of Oz". I always thought it was the best movie ever made.....Then, something happened, The Harry Potter movies came out and they were practically flawless production values.....focusing on quality....very similar to how they focus on quality making the James Bond movies.
For me it's 'Playing For Time' a 1980 movie starring Vanessa Redgrave. It has to do with the female orchestra at Auschwitz concentration camp and is based on actual events.
I remember watching that movie on TV. It also starred Melanie Mayron. I'm surprised she didn't win an Emmy for her performance.
There are several. I seldom watch a movie more than once, so if I watch one more than once, I have to really like it. I can't choose only one. I'd recommend most of these movies: https://www.imdb.com/user/ur52397356/ratings/?ref_=up_urrat_sm
I liked Shirley Knight's role as the SS guard Maria Mandal. I thought that she played that very well, she even looked like her it was uncanny.