Any fan of science fiction films will agree that time travel ranks pretty high in the annals of sci-fi plot lines. There's something about being able to go forward into the future, or backward into the past, that's intriguing for us stuck-in-the-present folks. For futurists, it gives them a chance to see what the world might be like hundreds of years from now, whether it's positive or downright terrifying. Lovers of the past get a chance to go back in time to experience a bygone era or right some wrongs, despite the largely held belief that changing something from the past can have pretty drastic effects on the present and future. Hello, McFly! While compiling a top 20 list like this is a matter of pure subjectivity, it must be done. If you don't agree with the picks, then there's only one choice: You must go back in time and thwart this effort. Or it might just be easier to send us feedback via our Facebook page.