Angela: Do I have to take the rest of my clothes off or can I leave them on? John: ... Angela: John? John: I'm thinking...
When Constantine busts into Papa Midnite's and is too impatient to read the card so he belts the bouncer and marches downstairs. Chas stands over the unconscious bouncer and giggles: "Who's the rat in the dress now, huh bitch?"
As Constantine ascends to Heaven, he flips off the Devil. Doubles as one of the most awesome moments in the movie.
It's important to realize this is as he's being dramatically pulled up to Heaven, and he does it in perhaps the most off hand manner possible, though it's complete with slow motion.
In the novelization, it is actually God controlling Constantine's hand, which is even funnier.
The very end of the movie, when Constantine punches Gabriel in the face instead of shooting him/her.
The funniest bits of the above scene are the couple of seconds it takes for pain to register to Gabriel (during which he whispers "wow" when he sees the blood from his mouth), and as Constantine keeps walking while he calls after him with his comments.
This moment in the car:
Constantine: Corinthians goes to 31 acts in the Bible in Hell. Angela: (in disbelief) They have Bibles in Hell.
Most of Satan's conversation with Constantine is hilarious, what truly sells it is the casual tone which John uses when talking to Satan.
Constantine: So how's the family? Satan: Family's doing fine. Busy, busy, busy! Need a vacation.
Angela's faint exasperation at not getting kissed three times in a row. It's blink-and-you'll-miss it, but watch her face each time Constantine leans in.
The scene in Angela's apartment. It's nice to see Constantine stop being mean and actually teasing her.
Angela: If this is some kind of spell, don't you need candles and pentagrams? Constantine: Why? Do you have any?
At the hospital early on, when the doctor is explaining that Constantine's lung cancer is terminal, and he lights up a cigarette.
Angela: I'm a Catholic, John. I know the Crucifixion story.
From the series:
In the pilot, Constantine's psychiatrist reads his business card out loud, emphasizing the "Master of the Dark Arts" line. Constantine looks up and remarks he should change it to "Petty Dabbler", as he does so hate to put on airs...
Same episode. He notices a rather large amount of cockroaches heading into the art room and towards a young woman who is painting something on the wall. As he approaches her to try to get her to stop, he notices her eyes are completely white and filmed over. He promptly turns away, muttering "No. No no no no no no. This is not my problem," only to stop, roll his eyes and say "Oh, bollocks." After he exorcises her, he realizes that he's got to leave and strides out of the room, tossing off a "She did it," over his shoulder as doctors and other patients rush in.
When he meets Liv, he first points out that she's holding her can of Mace the wrong way, then hands her a business card. When she points out the "Master of the Dark Arts" bit, he says exasperatedly "I'm getting new ones made!" Then he descends into the sinkhole, and when a dark shape startles him, he yells "Who's there? I'm a nasty piece of work, you can ask anyone!"
Constantine's way of getting in to fight the demon involves paying a security guard $500... Except he claims that he wants to have sex with Liv on the rooftop instead. He continues on to hand the guard a lightbulb, saying that he'll pay an extra $100 if the guard calls him the instant it starts to glow.