Billy Coen killing the Queen Leech with a single shot from the oversized magnum.
There's a decent stretch of game where Billy is unconscious (Eliminator attacks are nasty) and Rebecca must handle things on her own. During this time, she fights a Tyrant, in cramped conditions no less, and wins. Okay, so it was a somewhat defective Proto-Tyrant (so not at full Lightning Bruiser capacity), but the darn thing still had a Deadly Lunge that could go from one end of the room to the other, and claws as big as Rebecca's entire body, and poor, fragile little Rebecca turned out to be quick enough and skilled enough to bring it down (temporarily, anyway). For a character who'd been saved umpteen times by Billy, that was without question Miss Chambers' Crowning Moment of Awesome.
Even though Rebecca seems frail and weak, she's able to survive through not one, but two zombie games that happen within days of each other.