The best Algy moment comes near the end, when Biggles and some other people have left the plane to get something and left Algy with the plane (as usual). Don't want to give away too much, but suffice to say that people are shooting at Algy and he has taken off and flown off somewhere else.
Meanwhile, we have this conversation between Biggles and his companions:
"You're sure he'll come back?" Boris spoke with marked anxiety.
"If there's one thing certain in this uncertain world it is that Algy won't go home without us," returned Biggles curtly. "We don't leave each other in the lurch, whatever happens."
--Biggles Takes Charge
Not really an "Algy" moment as he isn't even there, but amazing and shows what kind of a person Algy is, so still counts as an Amazing Algy Moment! I love how Biggles doesn't even doubt that Algy will come back: he knows that Algy will always come back, no matter how bad the situation is.
Wish he'd remembered that he and Algy didn't leave each other in the lurch when he happily left Algy in the lurch and drove away in the stolen (?) cab after his meeting with Smith...
Even with the possibility of being shot down by his French colleague cannot dull Algy’s sense of humor, however:
Of course, only Algy could keep a cool (and amused) head under the circumstances. Bertie would no doubt be muttering darkly, Biggles would be wearing his famous fighting/mirthless smile, while Ginger…well…Ginger’s always been an easily excitable sort of kid. He’d probably do something crazy and end up wrecking his own machine as likely as not!