Alan Moore wrote a great movie called The Show which most people haven't seen yet.
The Thief and the Cobbler: The Recobbled Cut is an amazing animated movie unlike any other.
Ice Cold in Alex is a British WWII movie from the 50s about a medical field unit in the North African campaign. I don't think there's much combat in it, but it's a pretty great movie.
Lifeboat by Alfred Hitchcock is a 1945 WWII movie that is good. I say this as someone who isn't necessarily the biggest fan of old movies.
Film by Samuel Beckett is another old movie that people probably haven't seen. It's an Avant Garde short film starring Buster Keaton towards the end of his life.
Earwig and the Witch is the most underrated Ghibli movie. According to me, anyway. I think it's great.
Dave Made a Maze is an indie hipster comedy about a guy who makes a maze out of cardboard in his living room. Again, I think it's great.
Various obscure documentaries.
By using IMDB advanced search filtered for 1000-100,000 ratings, movies, English language, sorted by high rating, these are the movies in the first few pages I have seen and thought were good:
Fiction
Doctor Zhivago
https://www.imdb.com/title/tt0059113/?ref_=sr_t_431
Dramatised / semi-biographical
Non-fiction
Koyaanisqatsi
https://www.imdb.com/title/tt0085809/?ref_=sr_t_125They Shall Not Grow Old
https://www.imdb.com/title/tt7905466/?ref_=sr_t_126My Octopus Teacher
https://www.imdb.com/title/tt12888462/?ref_=sr_t_206
Using the same criteria as @TheAllMemeingEye of 100k ratings on IMDB or less, here are two I remember fondly from childhood. I won’t claim they’re cinematic masterpieces, but I still find them enjoyable.
Sneakers:
Drumline:
Homerun (Chinese: 跑吧孩子; pinyin: pǎo bà háizǐ) is a 2003 Singaporean Mandarin-language satirical period film. A remake of the award-winning Iranian film Children of Heaven, it is a film about two poor siblings and their adventures over a lost pair of shoes. Set in 1965, the year Singapore separated from Malaysia, and satirises the political relations between the two countries, leading to its banding in Malaysia.
That most people havent seen? Hmmm, maybe The Man from Earth? Not my absolute top 1 favorite, but definetly one I remember fondly.