I try to be honest and chose movies that I rewatched, quoted, and effected me the most. I know there are some all-time greats that might be better movies, but don't make the top 5 for me:
City of God
Gladiator
Dumb and Dumber
The Matrix
Mad Max Fury Road (tough choice over Road Warrior)
I reserve the right to edit later as I think of more.