which were darker and more mature

Well, some of the most immature things I've ever seen or read in my life were "dark" which is itself far too often used in various fandoms as a gauge of quality rather than a gauge of tone.

Out of the series that could considered "dark" or "mature" I would go with Kuuga because while dark it was mature enough to showcase the importance of having hope and joy even in the most awful times or when things seem bleak. That to become obsessed with it is to lose ones own humanity.