Portal 2 - The Part Where He Kills You

The player is put into this spike trap by the antagonist (Wheatley), and at this point the chapter text comes up saying "Chapter 9: The Part Where He Kills You", you get an achievement of the same name, and Wheatley then says "Hello! This is the part where I kill you!"

The timing and delivery of it was so perfect.

GTA Vice City - taxi and ambulance driving

I loved the part of VC (and I think other installments have this too) where you jump into either a taxi or ambulance and you can then become an actual driver for them, earning money. Loved that minigame for being such a different thing to all the other missions.

Driv3r - the Bascule Bridge

In one of the maps of Driv3r (kind of a GTA clone), there was a Bascule bridge you could actually toggle, and so I'd usually get a wanted rating, bait as much police and cars onto the bridge (even blocking the roadway with my own car) and then draw the bridge up with all of them on it, and watch how the physics bug out and some officers end up in water (should never happen in normal gameplay) and the cars just all explode in the water.