Crowley convincing his car that it still was a car, despite clearly not being so, holding it together through sheer force of will whilst it was on fire and utterly destroyed.
Also when he calmly asks for directions whilst his car is still on fire.
Oh, God, so seconded. And the fact that it's all told from the perspective of a bystander who's saying 'turn left, take the second right' and thinking '...does he know that his car is on fire?'
"It would take a lot to faze a copper from the Met. It would take, for example, a huge, battered car that was nothing more nor less than a fireball, a blazing, roaring, twisted metal lemon from Hell, driven by a grinning lunatic in sunglasses, sitting amid the flames, trailing thick black smoke, coming straight at them through the lashing rain and the wind at eighty miles per hour. That would do it every time."
First he takes out Duke Ligur by rigging his door with a bucket of holy water The same holy water that would presumably destroy him with just one drop, and that unnerves him just by being there even when locked behind more security than radioactive matter.
"...Holy water. You bastard."
Then he proceeds to take advantage of Hastur's paranoia and finally trap him in an answering machine. (He resists the urge to turn him into Freddie Mercury, though, because you can only go so far.)
With this done, he hauls ass to Aziraphale's bookstore, which is at this point on fire, and rushes straight into said burning building because he thinks Aziraphale is still inside.
Afterward, cue further asshaulery as he makes his way to lower Tadfield, utterly destroying his car until he's driving a fireball of charred metal and rubber.
Before even that happens, Crowley makes a list of things that are wrong...
1. Armageddon was under way.
2. There was nothing Crowley could do about this.
3. It was going to happen in Tadfield. Or begin there, at any rate. After that it was going to happen everywhere.
4. Crowley was in Hell's bad books.
5. Aziraphale was- as far as could be estimated- out of the equation.
6. All was black, gloomy and awful. There was no light at the end of the tunnel- or if there was, it was an oncoming train.
7. He might just as well find a nice little restaurant and get completely and utterly pissed out of his mind while he waited for the world to end.
Adam's three friends, all eleven year old children, taking out War, Pollution and Famine with nothing but sticks and strings. And Death's subsequent surrender.
Also when Aziraphale cast doubts in The Metatron and Beelzebub as to whether the Great Plan was truly Ineffable or even the actual Plan, convincing them to back off from Adam when he was weak.
Aziraphale and Crowley ready to fight Satan, finally assuming their true forms. What made it really awesome was that, while Aziraphale had finally picked up his flaming sword for the first time in 6000 years, Crowley was armed with a tire iron.
Shadwell joins them. When they tell him they'll be fighting the Devil, he tosses his gun away, removes his hat, and tell them he'll just headbutt the Devil into submission. He may be a loon, but he's an awesome loon.
Jaime Hernez the tree-loving janitor breaking a plastic window to let a mall tree grow, and laughing like a maniac.
Aziraphale and Crowley almost figure out the universe, or at least come close enough that some Laser-Guided Amnesia is thought prudent by the powers that be (whether the person speaking in small caps was God, or Death, or maybe they are the same, is irrelevant. They got close enough to make some people very nervous)