The death/defeat of any boss. The Forever Train is a particular favorite for some, when done correctly.
The team's journey through Area 6 can be considered one. Here you are, outside Andross's front door, and his army's throwing just about everything they have at you. But you just keep plowing through them. By the boss fight at the end, the enemies are at wit's end trying to find anything that can stop you.
The best part is when Rob actually gets the Great Fox to shoot some targets for you, finally getting in on the action.