Karone's reintroduction episode. Dressing up as Astronema and using her infamy to stop the auction of and walk out of the Bad-Guy Bar with the Pink Quasar Saber (which the Rangers had lost after Kendrix's death) and creaming everyone in a Bar Brawl when Trakeena exposed her, is just the tip of the iceberg. It was so awesome that the Fridge Logic didn't set in until the Humongous Mecha fight started and the plot stopped. It's a lucky thing that it and the subsequent episode were such great moments for her character, as, combined with the episodes needed to write around Valerie Vernon's illness, there simply was not enough time to really develop her character before the final Story Arc and finale.
Leo saved the entire season with one pivotal come-from-behind-victory. The key to his success? Ripping his shirt off.
Mike becoming the Magna Defender.
The last three episodes are filled with Trakeena CMOAs. (Yes, baddies get 'em too.) Suicide. Bomber. Mooks. And that's just one thing.
Villamax cementing his Noble Demon status by refusing to fire on innocent civilians and confronting the newly unhinged Trekeena. Sadly, it was his exit scene, but he did buy the civilians time to escape.
Of course, one of Trakeena's best moments was when Deviot pulled a Starscream on her for the first time. See, last Deviot had seen her, she'd just been Scorpius' bratty daughter, but was since trained by Villamax and Took a Level in Badass. So Deviot sends two assassins to do her in while she's fighting Leo (aforementioned Shirtless Scene)... and she owns them. After killing one of them, she pins the other against a rock with her leg (against its chest or neck, while standing on ground with other foot) and calmly demands to know who put her up to it.
...and then Deviot kills him, pretending to be trying to save her from the attack, before he can talk. Sometimes, his sneakiness could be awesome, too.
One of the final moments of the series is, in this troper's opinion, one of the most epic, jaw-droppingly awesome moments in the history of the franchise. The space colony (structurally, the ship is a collection of enormous domes connected by tubes) Terra Venture crashes onto the moon of the planet Mirinoi, and the passengers and crew successfully evacuate to the planet below. But they're still in danger - Trakeena, using her new-found power, raises the city dome (what it says, a dome the size of an enormous city) and throws it at the planet below. The Rangers arrive on board the falling city dome, fight and defeat Trakeena. But it's too late - the dome has made it through the atmosphere and is falling straight towards the colonists. From the size and speed of the city dome, the impact will kill everybody there and likely wipe out half the planet, and even the Rangers are panic-stricken and helpless. From out of nowhere, the Galaxy Megazord flies up to meet the city dome, grabs hold of it, slows the dome down and steers it away from the colonists and into an abandoned valley, giving the Rangers time to escape into their zords before the dome crashes. In terms of a power-feat from the Galaxy Megazord, it's astronomical. In terms of sheer epic awesomeness? Mind-blowing.
As far as Rangers fighting with their chief villain go, the last few episodes rank pretty high up there. Trakeena just throws out the Monster of the Week play book and throws everything she's got at Terra Venture and the rangers. Just the first episode in the finale alone has tiny rangers running from her huge starship, and Terra Venture being irreparably ruined. The second episode she rattles her entire mook army and has them swarm the accessory zords (save the original in the moment above) in suicide vests. This is a "Funny Aneurysm" Moment today, naturally, but even The Dragon is alarmed by her recklessness. So she completes her Villainous Breakdown by killing him, too.
Turning a nature-themed Sentai into a space-themed one on top of the behind the scenes disasters and still coming out with a damn impressive final product makes this series one for the production team.
Special mention goes to the episode "The Rescue Mission," which was essentially "Alien in 22 minutes on a Saturday morning kids show," involving very little morphed footage and no footage from the Sentai. And it is one of the best episodes of the entire franchise.