In Raven's Gate Richard's improvised Molotov Cocktail on the monster dogs. Keep in mind that Richard was never a fighter to this point, spending most of his life in his office as a journalist. To be able to so calmly form the weapon, under such pressure, with enough precision, throwing it at the monster dogs was pretty cool.
Evil Star
When the Old Ones are finally freed and Matt’s approaching the despair event horizon at the idea of facing them alone, he suddenly realizes he’s not alone. Using his and Pedro’s combined power, Matt wounds Chaos and sends his army running. This is where you realize if the Five ever meet, the Old Ones couldn’t even put up a fight.
Nightrise
When the past Five (except for past Jamie, who had been replaced by present Jamie seeing as he had sacrificed himself to trick the Old Ones successfully defeat Chaos.
Also, the sudden shift of Chaos' attitude from "Haha, you're still going to lose" to Oh, Crap! in a matter of seconds when Inti (Past Pedro) and Jamie arrive.
Necropolis:
When Matt climbs the sword ladder.
Scarlett unleashing a typhoon on Old One-occupied Hong Kong and obliterates it entirely. Terrifying? Yes. Awesome? Hell yes.
The finale in Oblivion, in which the British Royal Navy (albeit, by accident) nukes Hell.
Holly is just a Tag Along Kid with no special powers or destiny. Her reaction to seeing Chaos is to empty her pistol into him. It has no effect, but even so that took guts.
