Not sure if games count as software but if they do - Factorio. I don't play much these days but I'm still utterly stunned how a relatively small, humble team of developers can build something so robust and performant. Granted, I've never really tried pushing the limits, but not once have I felt like the game is even breaking a sweat while processing thousands of machines, belts, and bots. It's a miracle to me.

Their devblogs are really nicely written and you can tell they are extremely passionate about getting things right. In my experience that's a rarity now.

And of course, it's a brilliantly addictive game.