Those choices are so horribly narrow. Good titles('cept Iron Sky), maybe, but all of those are block busters. Watch some classics!
Personally for me, 2001 - A Space Odyssey(hard to watch though, but thoroughly intriguing), Star Trek I, A Clockwork Orange, Enemy Mine, of course the first Star Wars movies, No Country for Old Men, Pandorum, Sunshine, Moon, Apocalypse Now(and the redux version), The Hills Have Eyes, Seven Samurai, Titanic(don't judge), Ghost in the Shell - Standalone Complex(anime movie from 90's, mindfuck stuff), Full Metal Jacket, The Flags of our Fathers, Letters from Iwo Jima(last two are must-see's) and lastly Tinker, Tailor, Soldier, Spy.
Lots of sci-fi, but I love all good movies.
Also, selecting one is impossible!