Guessed by PenguinZ
This is what happens when you take an amazing little experiment and decide to turn it into a fully-fledged game expanding on everything that made the original good and not creating any new problems or gimmicks to ruin the experience. You get a masterpiece, and that's what Portal 2 is. There's nothing more to say about it - everyone here probably played it and knows how good it is, and to the ones who didn't, please do, because you're missing out.