Guessed by @Darashiva
This is such a classic. I remember being completely mind blown by it some dozen years ago, and it's held up on each time I've revisited it. It's got a great story, as great as a game like this can have at least, the single-player campaign flows in such a wonderful way and explores the idea of portals so deeply it makes its predecessor look like a demo (which, in truth, it kinda was). Then there's the co-op campaign, which I haven't played it since way back then with my brother, but it was great times too and I'm hoping to replay it sometime soon 
and as for the PC version, that awesome level editor, I can't believe how people make such amazing levels on it, there's basically endless Portal content and it's crazy that this game usually goes for about as much as a potato. Or maybe that's just fitting!
