Guessed by BraLoD
This game was more of a fun little experiment more so than anything, but as experimental, unambitious and short as it was, it was still as memorable as it gets. Portal barely lasted 4 hours in my first playthrough, but it was amazing throughout. All of the 18 puzzles were very well designed and made you learn more about all the stuff you could do with the portals while getting used to the mechanics, and after you're done with the puzzles the game sets you up for an awesome ending where you need to apply everything you've learned, but this time under pressure situations. There's not a lot more to say - each moment of this short game was very memorable.