Which science fiction movie..............?

Ok, so i am a bit of a sci-fi freak, yes i love sci-fi a loooooooooottttt!! And, no, unfortunately Darth is not my father =( (as my name is not luke!). So, here is my question, throughout the whole of the life of sci-fi in the movies, which is your absolute favorite sci-fi movie and why? Please put a why or don't bother to reply as your answer will be ignored and not considered for the 10 points. Thank you. I will reveal mine when i chose best answer, just so no-one can influence me or be influenced by me. Thanks again y'all!!