This is hard, but here goes, and in no particular order:

1. Requiem for a Dream: I watched it at 14. It was the first time I realized the immense impact film can have on a person, and also the first time I've cried because of fictional characters. It led me to both interest in film and slight drug-paranoia, especially the harder drugs.

2. Synecdoche, New York: a masterpiece of cinema. Every shot impeccably-crafted, and every actor incredibly emotive and expressive. This is, compared to Requiem, much more into the intellectually-stimulating end of the scale. Also very satisfying to re-watch, as you could easily miss out on some things.

3. Primer: speaking of re-watching, this one is just one big mindfuck. It's about time travel, but more about regret and the lengths one will go to amend what was broken. Warning: you will not understand what the hell is happening on the first viewing. Also not on the second.

4. Shame: the sophomore effort of director Steve McQueen, featuring the best performance of my favorite actor Michael Fassbender, deals with sex-addiction, pornography, and relationships in that context. Very sad, extremely beautiful.

5. 2001: A Space Odyssey: favorite from Kubrick's. I guess the only flaw in this one is the slow pace, and I guess this is not a catch-all recommendation, only for the patient.

These are just off the top of my head. Not exactly my top 5, but certainly five of my favorites.