Sailor Mercury using Shabon Spray to protect herself from a fireball in episode 45. She then sacrifices her own life to stop a youma from creating illusions.
Remember that time Mercury, who had pretty much no direct combat ability in the anime, walked off and faced the enemy (who outnumbered her four to one) by herself and destroyed their biggest weapon by hitting it with her computer?
Yeah, this was my favorite part of the episode, honestly.
The thing that killed me the most about this is how fucking calculated it all was. Everyone else very seriously considered that they might die on this mission, but Ami pretty much accepted that she was going to die after Makoto bit it, in no small part because she knew they had lost their powerhouse. She deliberately left Usagi with the next most-powerful members of their team and went off to try to do as much damage to the enemy as possible before she died, as well as buying them time to advance.
Sailor Mercury is a fucking boss.