Time Force - The Pilot, especially the scene where... the Red Ranger is slaughtered by Ransik
Lost Galaxy - Where the kid thinks he'll never see home again and regrets sneaking aboard Terra Venger, The Episode where Treacheron Battles The Red Ranger alone, The battle against Chilli-Fish (Leo is saved by his dead brother's dog tag and well.. night scenes in Power Rangers)
In Space - Countdown To Destruction... need I say anything more?
Wild Force - The one where the Conga Mega Zord is defeated, but the Soul Bird saves the original Mega Zord... really powerful scene where the Rangers get pwned only to have an old friend (the Wild Zords) save them.
Turbo - The Wishing Episode where Justin is forced to choose between Seeing His Father Again or Saving The World From The Centurion. When he has the last of Divatox's wishing coins.
I actually don't remember MMPR that well... but the scene in the White Ranger debut episode, where Tommy descends like an angel from heaven. Classic.
