If you've never heard of Opus Magnum then you should definitely read Bruno's excellent piece about why it's impossible, and why that's okay. It's a complex puzzle game about alchemy in which you construct mechanisms to turn atoms or molecules into whatever it is you're trying to produce to advance the story. It's getting rave reviews on Steam and I haven't heard anyone say a bad word about it.
If it sounds familiar then that's probably because you've seen a gif of it on Reddit or Twitter: watching those miniature machines swivel and twirl is immensely satisfying (again, there's plenty to watch in the article linked in the previous paragraph), and at the end of each puzzle the game encourages you to share your clips.
It comes with a story-driven campaign, a puzzle editor and Steam Workshop support. Anyway, the motivation behind this post is that it's just left Early Access after two months of player feedback, and you an pick it up with a 10% discount on Steam, where it costs £13.94/$17.99.
It's Steam only for now—GOG didn't fancy selling it because it "looks too much like a mobile game", developer Zachtronics said on Twitter. That seems a rather strange decision.
If you enjoyed Zachtronics' previous games, including Infinifactory and SpaceChem, then it's definitely worth considering.