I actually sat down a coworker years ago and did this. It was surprisingly easy once I realized something - I'll sit down and watch a lot of movies on TV, but there are a few I refuse to. Too many commercials and edits just make me furious... those are my favorites. Getting down to 10 was hard enough, so top 5 are in no order. Some changed how I thought about things, some just entertained me for years:

The Matrix

Interstellar

Monty Python and the Holy Grail

Fight Club

Schindler's List

And because it was so hard, here are a few more that just as easily could be top 5:

Silence of the Lambs, Inception, V for Vendetta, OG Star Wars trilogy, Indiana Jones trilogy, Kung Pow, Blair Witch Project

Edit: Forgot one - Arrival. Probably in top 5, no idea which to demote. Fight Club maybe