Destroying a Reaper
Mass Effect 1
Saren is a huge dick throughout Mass Effect, from his betrayal in the very first mission to his reincarnation as an undead, Sovereign-controller avatar on the Citadel during the final boss fight. But his defeat leaves the Sovereign wide open to attack, and it's ultimately through Shepard's intervention that the fleet is able to destroy the Reaper.
Naturally, the Normandy fires the killing shot, buying the galaxy a little more time to prepare for the full scale Reaper invasion. Too bad they squander it, though it'll make Mass Effect 3 a lot more exciting.