Shunning the huge universes and myriad alien races of many other sci-fi games, Portal opted to focus in on a rather more confined snap-shot of the future. Depicting protagonist Chells battle through the ingenious puzzles of Aperture Science, the Portal series ground-breaking gameplay was unlike anything gamers had ever seen before when it was released back in 2007. As well as the pioneering puzzles, Valves game was praised for its effective use of humour and rewarding learning curve. The originally Portal may have been the pioneer but Portal 2 upped the ante in practically every single area, warranting its place on the list. A more ambitious narrative and more diversity in the locations combined with excellent new gameplay additions like the light bridges, gels and lasers meant that Portal 2 certainly outshone its predecessor, while keeping all of the elements which made us originally fall in love with the game. It may not be the sci-fi experience which completely immerses us in an alien universe like some of the other games featured on this list, but the futuristic hints combined with the gameplay means that Portal 2 is deserving of its place here. Moreover, the series antagonist GLaDOS eerily voiced by Ellen McLain is one of the finest baddies in all of science fiction, and could even give HAL 9000 a run for its money.