Factorio: Space Age Review (Hex)
Expands the base game's content by about an additional x2-3. The quality is superb; it isn't content bloat. Every item has a meaningful method of acquisition, use, and cost-benefit factor. It doesn't deviate from the core of what made Factorio engaging but expanded on it in multiple interesting ways.
The dev team consistently released detailed and usefully informative dev blogs about actual game dev including development tools, development techniques, concepts, iterations, thoughts, reasoning, and previews. This helped greatly inform the player what they were buying, without spoiling all the content ahead of time.
I'm blown away that this DLC is even a DLC, as this could easily have been sold as Factorio 2 for double the price and I would still think it was justified in value. As a consequence of this masterful game, Wube Software has earned a shortcut to my money and attention for any future releases. When I say they've earned it: I mean it.