Jim's Roaring Rampage of Revenge near the end - particularly the parts involving him taking a guy out with a crowbar and basically skewering a guy through stomach on his own bayonet. The whole thing. A Crowning Moment of Awesome that also happens to double as a scary mental breakdown.
Hannah's CMOA is less gruesome than Jim's, but her backing the cab through the front door of the mansion so that West would be torn apart by his own infected soldiers definitely counts. Double props because she was high as a kite at the time.
Selena reviving Jim at the end. He was shot in the chest, the hospital had been abandoned for God knows how long and she only had a teenage girl to help her. Girl's got skills.