Hate him all you want, but the moment where Rico impales an elite Helghast inbetween the eyes with a javelin gun from another moving aircraft more than qualifies in Killzone 3. The fact that they show the impaling in slow motion and rotating camera definitely amplifies the crowning awesome of the moment.
The scene where Sev mocks Visari in front of two Helghast soldiers while being captured deserves some mention.
Sev: "Your King went down like a pussy!"
But the best one of all has to be Sev and Rico's Big Damn Heroes moment in Stahl's factory! They enter the factory disguised as two Helghast soldiers and make their way over to the execution room where Stahl plans to kill Narville. Then, right after Stahl's amazing Rousing Speech, The two ISA soldiers take off their disguises, shoot up the place, and rescue Narville, complete with Sev saying a Pre-Ass-Kicking One-Liner!
A multiplayer version of this comes from the ISA victory cutscenes in Operations mode. If you win the match as the Helghast, all you see is a group of Helghast soldiers rounding up enemy survivors and preparing to execute them. But, if you win as the ISA, you get to see an epic cutscene of you and your team blowing up a dam, stealing a tanker, or hijacking a MAWLR, all with Narville praising your efforts as you watch!