Power Rangers In Space.
Not Mighty Morphin, In Space.
No matter what any one says, In Space was one of the; if not the; most well written season of Power Rangers. You have an actual relationship built up from the Andros and Ashley that sadly never had them kiss, but they went out on dates and showed true affection for each other. You had the DEATH OF ZORDON. You have the abduction of Andros' little sister when they were like 6. You have the Psycho Rangers [Fucking amazing sub plot there.] I rewatched this whole season recently and wow. There have been darker moments in Power Rangers, especially in later seasons, but this is hard to top by over all story.
Oh, and if you want examples...Lost Galaxy, Magnum Defender's son is killed on screen. In Time Force, for the longest time you are left to believe the Red Ranger is killed. In Dino Thunder, the Villain has a Dr. Jeckle and Mr. Hyde issue, similar to his son. Lost Galaxy, a ranger actually DOES die, giant explosion, ghost and all. And lastly, in RPM, a child and an old man die.
But yeah, that's about it. Anything between Dino Thunder and RPM sucks in my honest opinion, but maybe I've just seen really terrible episodes of those seasons.
Edit: Also this opening is awesome.