This series does everything perfectly and the ending is no exception. It wraps the entire series up into a perfect paradox that makes the viewer feel like what happened in Jonas' world could be the reality of ours and has the same kind of feeling as Inception does. Like other comments have said, this is a masterpiece.
It is clear that the writers had the entire series in mind when they started season one, which makes for a perfect finale. It would be great if more series were designed with the precision that Dark clearly was. From the perfect loops that tie everything together, to the hidden foreshadowing in the first seasons, to the interesting allusions, this series is definitely worth watching... twice