George's showdown with Herrick, with George embracing his wolf side to frightening effect to tear a defiant Herrick to pieces
George "You shouldn't have gone after Mitchell. It got my... attention."
Pretty much all of George's action in that episode, manipulating both Herrick and Mitchell and by extension all of the other Vampires in Bristol. He finally matures and puts aside his Non-Action Guy demeanor for long enough to kill Herrick and save Mitchell and Annie. Sometimes I wonder why people assume Mitchell is the true hero in this show...
The McNairs' and Mitchell successfully taking on a room full of vampires
In a similar vein Annie, George and Lauren in the funeral parlour
Mitchell charging into the gates of Hell ready to fight God and Satan at once to save Annie.
Arguably Lia's What the Hell, Hero? rant to Mitchell, when she calls him out on murdering herself and the train full of people, and then trying to just ignore or make up for it with good deeds.
Nina bailing George out of prison, right as they both are transforming. The fact that she more or less keeps a straight face and gets them both out of there while beginning to suffer a heart attack, kidney failure, and other forms of Body Horror is impressive.
Facing half a dozen policeman armed with assault rifles in his kitchen, Mitchell brandishes a jagged bottle handle at them and tells the police they have one chance to leave, or he'll kill every one of them. Since Mitchell's not given to empty threats, you start to suspect he could actually do this.
After Mitchell is arrested, amnesiac Herrick reluctantly succumbs to his Blood Addiction, causing his memories to return (and thus his original personality) while the house was still full of heavily-armed police officers. Cut to a few minutes later, and the officers are all shown to have been brutally slaughtered, with Herrick even stealing one of their uniforms. This implied Curb-Stomp Battle cements Herrick as one of the most powerful Vampires on the show.
Tom effortlessly killing Herrick's henchmen - including snapping the neck of one - and then flattening Herrick almost killing him before Mitchell stops him
George saving Mitchell at the end of Season 3 by killing him. Doubles as tearjerker His Badass Boast To a freaking Old One! is also pretty awesome
George: It looks like you've got a fight on your hands
George saving his daughter Eve from vampires by willingly inducing a partial transformation by focusing on a picture of a full moon. He then goes on to slaughter a room full of vampires, helped by a stake-wielding Tom. His Heroic Sacrifice then makes this a Tear Jerker though.
Tom, Hal and Annie working together to take down a room full of vampire in Season 4, Episode 3. It's the moment they really become a team.
Annie returning after disintegration, eerily glowing with a blue light, and killing Kirby to protect her adoptive daughter.
Althought it's all just a dream, Hal shows in 2 seconds why people should fear the Old Ones when he effortlessly kills Tom with his own stake. The latter had been shown to be one of the most capable combatants on the show, but Hal offed him without breaking a sweat.
Annie taking on a room full of Old Ones effortlessly, then making the ultimate sacrifice to save the world. The look on Mr Snow's face when she storms in is priceless.
Annie: "GIVE ME BACK MY FUCKING BABY!"
In a macabre way Cutler's Death. Throughout the series he had been either a Manipulative Bastard or a Smug Snake but when it came to His death, he was a pure Determinator. His Rasputinian Death was undeniably awesome: burning alive by walking into a house uninvited in order to kill Eve and wipe out his own species, and then being staked by a stake thrown telekinetically thrown by Annie.
Hal giving an epic Reason You Suck Speech to the werewolf Larry, who's stolen from Tom and destroyed his self-esteem. He then gives him five minutes to get out of the house before he badly hurts him. Larry, who's both a Jerk Ass and a complete idiot, tries to provoke Hal into fighting him, which he manages by calling Alex a whore and Tom a retard. Hal then proceeds to strangle him with a lamp cable, in a combined moment of awesome and dose of nightmare fuel, tinged with Tear Jerker.
Crumb staking Alan after he tries to drive him to drink blood again, then sitting on the opposite side of the room from the cups of blood.
Tom pulling the greatest Big Damn Heroes the show has ever seen. He runs to the hotel from the woods while transformed, then rugby tackles the werewolf Bobby into a room to save Hal.
Hal similarly saving the hotel's residents from Bobby, using nothing but a chair and himself as bait.
When Rook and two subordinates search the site of a massacre at a vampire blood farm, Rook finds a small girl and takes her into his arms. At that point vampires leap at his two co-workers, in a rare display of super strength in this show. Rook just calmly pulls out a large cross while holding the girl's face to his chest, telling her "don't look at the monsters".
When Captain Hatch rises. He gets up out of his wheelchair, grabs Alex by the throat, reveals to her "I'm the fucking Devil, sweetheart", then throws her through a mirror into her own grave.
In the finale, Rook shoots the Devil in the face. When that doesn't work, Tom, Hal, Alex and Rook perform a four-species tag-team Heroic Sacrifice to render Satan himself Killed Of For Real. As it turns out, it was only fatal for Rook.
Alex phasing through her coffin and then crawling out of her grave.
Alex: I'm a fucking X-Man!
Hal and Tom's brawl in the finale was a CMOA for both of them. Tom for staking and Werewolf Blood-ing his way through an entire team of Vampires, and Hal for gaining the upper hand in a fight against Tom (who is one of the most capable combatants in the series). Even when it looks like Tom's won, it turns out that Hal realised that Alex had arrived, which is why he stopped fighting back.
The Trinity rejecting the Devil's offer of dream realities where they can be human. Alex refuses to let her real family suffer while she enjoys time with a fake one, Hal sees through the fact that the Devil is tricking him despite being distracted by a mortal stomach wound and a fake Leo, and Tom rejects the Devil's claims that his friends care nothing for him, and stakes him just to make a point.