The end of the Metallia-possessed youma. The thing was pretty much undefeatable, and was about to kill Sailor Mars, Mercury and Jupiter. Minako, who was almost dead from her illness and had been unable to transform, arrives, take a look, and not only finally awakens her full power, but her first attack is so devastating that it kicks Metallia out of the youma. The youma survives, and is still powerful in its own right, but after that the Sailor Senshi, now joined by Moon, finally kills the thing with a combo attack.
In the episode Minako dies, Moon, Mercury and Jupiter are fighting a bunch of low-powered mooks and a more powerful one, and risk being overwhelmed. That's when Rei shows up. Rei is initially dazed, as she has just found out by Artemis about Minako's death, but then Jupiter snaps her out of it... And she burns down the main youma before starting crying in grief.
Also from the Special Act. Endymion and the revived Shitennou were having an hard time against the Youmas Sword and Shield, the first having immense attack power and the second being pretty much invulnerable and protecting his colleague from retaliation. Cue Endymion tricking Sword into hitting Shield with enough force to kill him while breaking his hand. An Oh Crap later, Sword is dead too.
Ami'sTransformation Sequence in Dark Mercury and Minako's in Sailor Venus are two for the production team: Usagi's transformation in Sailor Moon, the first they created, was rather sloppy and cringe-worthy, but by the time they got to Venus and Dark Mercury they had a lot of experience, and it showed: Venus' was the most complex and shiny but flowed magnificently, highlighting both her role as leader of the Senshi and fake princess, while Dark Mercury's, while much simpler, is just as magnificent if not more.