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.
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.
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.