Fowler infiltrating the bank robbery in "Rag Week" by disguising himself as the pizza delivery boy the police had called in, then calmly ordering them to turn over their guns. (Of course, he thinks they're just university students pulling a Rag Week prank, but still.)
Patricia dealing with the catcalling builders in "Come On You Blues"
[As Patricia goes to walk into the bank, the builders start catcalling.]
PATRICIA: You want me to get them off? [noises of assent] Right! [She opens her coat, revealing her police uniform] You're all under arrest!
Fowler getting back at Grim for charging Habib by raiding the lock-in, topped off by Fowler waiting in the bathroom for Grim and Boyle to try and escape.
FOWLER: The problem with old coppers' tricks, Constable Boyle, is that old coppers know 'em.