2001: A Space Odyssey
Sure, it's a classic, but it's not perfect. People have complained over the years that the confusing and difficult plotline has made it age poorly. However, the abstract nature of the movie makes it both compelling and terrible, so proceed with caution when watching this.
The Lord of the Rings: Return of the King
Adapting a novel to the big screen is no easy task, and brings in some critical reviewers. The end feels like the cast and crew saying their goodbyes to each other instead of a close to a character's life. Apparently, the director's cut was even harder to watch...