I think the storyline arc with Power Rangers in Space was the best of the franchise.

They really did a fantastic job building Astronema as a dangerous, but sympathetic villain. She was certainly malicious in her actions, but clearly not entirely evil like some of the other villains. I mean, she goes on a date with the Silver Ranger in one episode. How bizarre was that? The whole mystery surrounding her is slowly unveiled up until the halfway point or so when its finally revealed that she is indeed Andros's lost sister. Shortly afterward, they actually tease that Astronema is going to become the first Purple Ranger. And she's the main villain in the season! That's insane to think about, fellas.

I was always touched by Ecliptor sacrificing himself to save Astronema after she turned face and sided with the Rangers. He was a father figure to her, and despite Astronema's decision to help the enemy, his instinct was still to take the bullet for her. I found that to be a very heartfelt moment.

It was a great story arc and really added a level of depth that hadn't ever been seen in Power Rangers before. But with that being said, I haven't sat down and watched a full season of Power Rangers since the early 2000s. Dino Thunder was the last, I think. Maybe there've been better seasons since then. Not really sure though.
R.I.P. Bert