Power Rangers In Space
In Space was the first truly mature season. It had complex and edgy storylines such as Zhane and Astronema hooking up, the search for Zordon, the main villain being Andros' sister, and the final battle Countdown to Destruction. For me, In Space is the most epic season of Power Rangers.
Lost Galaxy was also a very mature season and was pretty dark. Lost Galaxy had more on screen deaths than any other season of power rangers and also had the morally questionable Magna Defender. Overall, I felt Lost Galaxy wrapped a nice story together, and did better than any other season of mixing the Sentai plot with its own plot. Still, for me Lost Galaxy isn't quite as good with the drama as In Space and RPM are.
Lightspeed Rescue was another mature season, but not as mature or as dark as some of the other seasons on this list. Its maturity mostly revolved around the character Ryan, whom was Captain Mitchell's son and Kelsey's brother. The story involved Ryan being evil at first and later having to fight for his own life, which was threatened by the cobra tattoo that was placed on his back by Diabolico.
Time Force is another dark season that dealt with issues such as death and terrorism. However, most of the plots surrounding these issues were copied heavily from its Sentai counterpart, Timeranger. The only real mature issue that was original and Time Force covered was discrimination against mutants in the future. This was shown with Ransik's backstory.
SPD I felt had some really mature situations as well. Particularly surrounding the murder of Sky's father, the kidnapping of Crugar's wife, and the destruction of his home planet. It also had two Robin Hood like characters, Jack and Z, who were offered to join the team as redemption for some of the crimes they committed previously.
RPM is probably the most mature and darkest season of Power Rangers. With the setting of the post-apocalyptic world, as well as many of the character's backstories. Particularly noticeable was Dr. K's backstory, how she was basically quarantined inside a military bunker her whole life and how she was the one who created the Venjix Virus that went on to nearly wipe out humanity.