Mine is probably in Fable III when you go fetch the guy's ring that he dropped in the sewer. In a world that at its best is apathetic, and at its worst simply sadistic, what point is there at getting married?
Ah, I killed all those mercenaries and hobbes for no reason at all...
Mine is the last mission from Halo 3. Yeah, it might be unoriginal, but the escape sequence, when you barely get away from the Halo ring in the Warthog (or Ghost), is just so epic. Love it.
