You've gone through hell and back, through a demented apartment complex haunted by a pedophile, a city crawling with monsters, a train station full of nightmares, a trainwreck, a dark forest with that damn chainsawer, a mental ward, and all of the various challenges and threats inbetween. And then you finally make it to Simon's hometown, the sun shining. Even when enemies still appear, you feel less like you're merely wandering around blindly or praying to survive, but.. Strong. Empowered, at the end of a long and tiring journey.
Watching Simon crush the doctors skull with his foot.
The best ending. Simon literally kills the Anthropomorphic Personification of his desire to commit suicide. With a pistol. While confined to a wheelchair. And the personification of said desire had a shotgun, a machinegun, and an axe.
While the canocity of it is questionable at best, the Cops deserve a mention in the Co-op campaign for being able to survive the hordes of monsters and saving Simon from the drunk driver, preventing the Book, and thus the monsters, from ever threatening the world.