Guessed by @TruckOSaurus
They had a great concept in Portal - and with Portal 2, they had a great game.
There's nothing I can really fault this game for. It's just amazing, really amazing, all of it. Gone is the prolonged tutorial of the first game, here you get your double portal gun early doors and from then on it's just straight up puzzling, with a considerable amount of levels meaning you're gonna spend some serious time going through it all. The puzzles are creative, fun, explore all sorts of cool gimmicks, and, best of all: when you think you beat the game and that's that, you still have a whole co-op campaign to plow through, and a level editor meaning there's potentially infinite content here. The story is pretty cool too, quite minimalistic but extremely effective. Portal has a certain vibe to it that is extremely unique, and I'll always remain fond of this game.