In the very first chapter, Lilith's Dynamic Entry. She's descending toward Eden Prime in an escape pod that's getting shot at by an Atlas gunship. Rather than let this get her killed like most sane people would, she leaps out of the pod, Phasewalks into the gunship, blasts it so hard it drops out of control, then jumps off, Phasewalks to the surface and lands in front of two startled Lance troops. Then, she takes off her helmet to reveal both her Power Tattoo and Supernatural Gold Eyes, stares at them, and comments "'Sup."Then the gunship explodes behind her.
Brick's Big Damn Heroes moment to save Doctor Michel. Bit deflated by the fact that she seemed to have it under control, albeit in a screaming, swinging buzz-axe kind of way.
Tali's introduction. Brick is setting out to rescue her from Chora's Den, and is all set and ready to bust in guns blazing. then Tali blasts open the door to the Den, riding a giant, heavily armed mecha-skag, one-handing a shotgun at Fist's goons.
Mister Torgue unveiling his new corporate headquarters on Illium by blowing up the top ten floors in the shape of a giant flaming middle finger aimed at the headquarters of Hyperion and Maliwan, all the while playing air guitar on live galactic television.
Udina's introduction in the second chapter has him deftly outmaneuvering the batarian ambassador's accusations of acts of war. Then in chapter five, Udina appears in person by grabbing Nine-Toes' weapon out of his hand, punching him into the air, and blowing him up with a Torgue-Urdnot pistol in midair.
In the first chapter, one of the Reapers Phasewalks. That's right, there's a Reaper Siren.
Legion/Geth-Buddy/Gethtrap implies that its not just a Reaper Siren. Its a Reaper- Siren-psycho.
Oh, it's no longer implied. Not after the reveal that Saren is Krieg (with Sovereign as the crazy voice).
The arrival of the FISTING BOOMING SHITPUNCHER. It swoops in to rescue the Decisive Deception over Illium, charging into an entire Maliwan flotilla and smashes them flat, ending the encounter by ramming the last Maliwan ship with a giant fist-shaped ram mounted on its bow covered in explosive panels. Then it does donuts amid the debris field.