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Tear Jerker: Mighty Morphin' Power Rangers
The end of "The Green Candle", by the end the candles' already burned out despite everyone's best efforts and Zordon explains that the only way to prevent the Green Ranger power from being stolen is for Tommy to give the power to another Ranger. So Tommy chooses Jason, who gets the Dragon Dagger and shield. Tommy demorphs. Tommy puts on a brave face but you can tell the failure to protect his Ranger powers hit him pretty hard.
It's just as hard for Jason, who blames himself for not getting the candle when he had the chance. And it haunts him for a long time until "Missing Green".
From the "Green No More" two-parter, the scene when Goldar appears and then shows the already dejected Tommy footage of when he was the powerful Green Ranger (some of it being from when he was still evil, thanks Goldar).
When Rito destroyed the Thunder Megazord Season Three episode "Ninja Quest", that was the last time we ever got to see the original Dinozords in action.
All the times that it appeared that the Zords had been destroyed. While Word of God has said that the "Mighty Morphin" Zords aren't sentient like their Japanese counterparts, those cries sure seemed to have a lot of emotion in them.