The moment where after making a Heroic Sacrifice that simultaneously saves the world and rescues an innocent soul from Hell by killing himself to summon Satan to collect his soul, said sacrifice causes Constantine to start ascending to Heaven... and flip Satan off while doing so.
What about Lucifer reaching into Constantine's lungs and pulling the cancer out? That was pretty fucking special.
Lucifer demonstrates just why he is the boss of his domain by effortlessly dealing with Mammon.
John and Chas taking out a room full of demons by first blessing the water in the sprinkler system of the building, then setting it off with a lighter, than taking the survivors down with shotguns loaded with holy bullets.