Never in my life have I seen a game as optimized, thought-through, engaging and just overall well-designed as Factorio. Every single mechanic fits together like a finely tuned cog in a machine.
No other game manages to create a gameplay loop as engaging as Factorio as everything is tied to your own actions - the only problems you will ever encounter are those which you create yourself, thus you will finally realize: you are your own worst enemy.

And through extensive mod support perfection is even improved upon.

I am very picky in my taste of games, but for me, Factorio is truly without flaw.