While the footnotes are hilarious just by themself (no doubt because Bartimaeus is quite the smartass), the fact that they're justified in-universe (Bartimaeus' mind works on several levels of consciousness, so this is his way of putting that on paper)
One of Bartimaeus's enemy djinn for a single, brief instant revealing its true form when in a crowd of crows. Some freeze in pace, some drop dead, and the rest scatter. Also, before that, a female crow trying to court Bartimaeus when in crow form.
Jabor bumping his head on the doorway while chasing Bartimaeus and Nathaniel.
Nathaniel's hilariously subverted attempt at a dramatic reveal in front of Kat in book two. At first, the entire scene feels like it was lifted from a mystery anime. He brushes his Peek-a-Bangs out of his face, but Kat doesn't recognize him.
Duvall's death. After being arrested, he kills his guards as a werewolf and leaps out the window. So he escaped? Nope, they were five floors up.
Maybe it's just me, but the mental image of a djinn taking the form of a footstool is sporfle-tastic.
There's also the fact that it turned out to be an actual footstool...
Bartimaeus: (talking about Honorius, an insane rogue afrit) He's not going to be the only mad one if we set this lot loose. Look at that djinn over there. Took the form of a footstool. You know, it's weird, but I think I like his style...
Nathaniel: That is a footstool, Bartimaeus. Nobody is using that pentacle.
Hell, Bartimaeus is pretty much a walking, talking Crowning Momentof Funny himself. There's usually at least one in each of his chapters, especially in the footnotes. A good example is him unknowingly using the Rosetta Stone to bludgeon a golem.
Even more funny if you know that a Ptolemy wrote the stone anyway. A relative of Bart's Ptolemy.
Bartimaeus beating up two people... as a field mouse. Made funnier by the fact we don't get to see exactly how he did it.
This gets something of a Call Forward in the prequel, where Tybalt beats up Gezeri as a green-eyed white mouse, but how he did so isn't shown.
The aftermath of Solomon trying to use the serpent statue on Khaba, and immediately going for the anti-theft mechanisms.
The conversation Nathaniel and Bartimaeus have after Nathaniel is knocked out by trying (and failing) to activate Gladstone's Staff.
Bartimaeus: The magical energies have been gradually ebbing through your system. Your skin's been steaming and the end of each hair's been glowing at the tip. A remarkable sight. Your aura's gone haywire, too. Well, it's a delicate process, ridding yourself of a charge like that. I wanted to wake you straightaway, but I knew I had to wait several hours to ensure you were safely recovered.
Nathaniel: What?! How long has it been?
Bartimaeus: Five minutes. I got bored.
Why is there no love for Nathaniel not realizing he was being held back from Gladstone's staff by a "push" door?
"I know the secrets of the earth and the mysteries of the air; I know the key to the minds of women. (Patently all lies. Especially the last bit.) What do you wish? Speak."
Even funnier once his summoner (Ptolemy) starts asking questions about the nature of demons- which Barty has no idea how to answer. He's so stumped by this line of questioning that his current form (a sand tornado) freezes in shock.
During Bartimaeus's 'visit' to Pinn's, Simpkin mentions cleaning Nefertiti's anklet, meant for the Duke of Westminster's wife, and claims that it bestows great beauty on the wearer. Bartimaeus, who had procured it for her and knew that she was already quite beautiful, mentions that it actually forces her husband to obey her every word, and wonders how the Duke is managing. Two chapters later, Nathaniel encounters the pair briefly - and the Duke is simply described as "exhausted-looking".
In The Ring of Solomon, Bartimaeus's constant quips regarding Asmira's inability to think for herself.