Gaspode and Laddie's (Good boy Laddie) near-Death experience, with a maimed Gaspode squaring off against Death to protect Laddie.
The entire romantic sub-plot involving Ruby and Detritus. Even if Detritus' attempts to woo Ruby don't always go down very well, it's still kind of sweet to see that even trolls have gentle sides.
Especially when viewed in light of Detritus' wider character arc. He started as the archetypal, barely-sentient troll extra. These days he's one of the Watch's finest officers and he and Ruby recently adopted a young troll, Brick. His courtship of Ruby in Moving Pictures is the moment his life turned around.
Ginger's last line to Victor at the end of the book. "Cheer up. Tomorrow is another day."