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.
"The name of that is pain. Get used to it."
From the series
Non Est Asylum
The demon Furcifer tells John he can't trap him because he could draw in the electricity from around them and John lacked the "Power of Intention". Cue the power grid being shut own and the magic circle being set on fire via lighter toss.
Furcifer: What's this? John: The Power of Intention.
And John finishes off by giving a Badass Boast (see main page) before banishing Furcifer back to Hell in a pillar of fire.
"My name is John Constantine. I am the one who steps from the shadows, all trench coat and arrogance. I'll drive your demons away, kick 'em in the bollocks, and spit on them when they're down, leaving only a nod and a wink and a wisecrack. I walk my path alone, because let's be honest, (Cue Hands of Fire) who'd be crazy enough to walk it with me?"
The Darkness Below
The priest, who throughout the whole episode has been depicted as a pathetic drunk, figures out the missing piece before Constantine or Zed.
Constantine turning the tables on the episode's antagonist by invoking the spirit of her dead husband, who also was a miner, to drag her to hell
The Devil's Vinyl
Near the end of the episode, where John Constantine storms the radio station while "Anarchy in the UK" by The Sex Pistols plays in the background (and into his ears on an mp3 player). One of the most awesome uses of music in a network TV series in recent memory.
Followed up by casting an exorcism on the cursed record, sending its evil back to Hell and taking Midnite's mooks with it. And then in the episode's denouncement, he forces the soul broker to break the deal for Jasmine's soul by eating their contract — literally.