Colonial!Aidan completely losing it and slaughtering dozens of people that were attempting to drown his wife for allegedly consorting with the devil.
The second episode of Sally's alternate timeline opens with Aidan coasting down a sloped hallway on a stretcher like a scooter, listening to music on an MP3 player. It's a relatively quick and trivial scene, but it's awesome to watch.
About everyone in the Boston werewolf pack come to the house and threaten Nora at gunpoint in an attempt to capture Josh. Josh's response? He wolfs out and rips every single one of them to shreds. Even though most of this happens offscreen, the followup scene in which Aidan and Sally come home to a house full of gnawed, dismembered corpses really hits home, especially when we see Josh in human form with a thousand-yard stare, covered from head to toe in blood and being comforted by Nora.
From the finale: Sally has pulled a Heroic Sacrifice to save Josh from getting killed by Aidan by taking Aidan's vampire side at the cost of her own life. Unfortunately, Aidan's body starts to rapidly age, with Nora giving him a week at best to live. Rather than spend it moping around, Aidan goes back to the house and the spirit inside of it that has been tormenting the team throughout the show. After she gives Aidan a Curb-Stomp Battle (Not hard, since the guy's aged (Within a day) to the point of having difficulty going up a flight of stairs) and tosses him off a flight of stairs (Similar to how Sally died, except Aidan chose to fall off), Aidan, in his dying moments, picks up a lighter and tosses it... lighting a gasoline trail on fire that takes the whole place down. Soon after, Josh and Nora find Aidan (Now a ghost), waiting for them and ready to get back to having fun with them... until his door shows up, he goes through and sees Sally, waiting for him. That's how to end a show with a bang.
Even better- Being Human ended several weeks after How I Met Your Mother ended... controversially, with the fanbase split down the middle. With this show? Virtually no-one's complaining.