For Looking Glass, more like Crowning Level. Just mention the Thief II level Life of the Party to any Thief fan and they'll grin and remember the first time they played that huge, very open ended and incredibly well designed mission that combines burglary via rooftops of noble mansions, banks, a necromancer's spire, and dingy tenements with Garrett's gatecrashing of that party. And Karras knew you were coming ALL ALONG! Let's not forgot what happens after you read the Karras' New Scripture... arguably the biggest scare of the entire series, though you weren't in any danger!
The way Garrett defeats the Big Bad in all three games: He uses nothing more than his expert knowledge of stealth and subterfuge.
Thief: Deadly Shadows ends perfectly, bookending the series with Garrett now as the Head Keeper capturing a girl who reminds him a lot of himself. His smile doubles as a Heartwarming Moment.
In the first game a crime boss named Ramirez tries to have Garrett assassinated for being an independent operator. Garrett decides to send a message by breaking into his mansion and lifting the purse off his belt, with no preparations beforehand. Even better, at the time Ramirez is in the vault in his basement merrily counting his money. The message is clear and powerful: If Garrett wants to get to you, nothing you have will stop him. You will never see him coming. Do not mess with Garrett.