In a mission for BF3, I would use the Fus Ro Dah button when the enemy soldiers try to get a piece of me when my tank is busted and there are too many guys to kill.
When the guys in inFamous all have AK-47s, but can hit a flea on a dog from 200 yards and all of them are running around like drunk drivers but still continue to completely own you.
Or when a swarm of Yanme'e attack you on legendary mode on Halo.
Oooooooooooooo!!! I have one!!!!!!!!!!!!!!!!!! Why am I acting like a little kid???????!!!!!!!!!!!!!!!! Ok ok ok! When you're almost friking dead in splinter cell conviction! AHHH!!!!!!!!
Any game that has jump scares. Games such as Amnesia, (I know gunsofliberty already suggested this) Penumbra, and Slender. There are plenty more out there, but these are the ones I can think of right now. Also, Assassins creed II. If you could FUS RO DAH those stupid lute playing entertainers without any guards noticing, it would give me a disturbingly large amount of satisfaction. I would probably FUS RO DAH the Guards as well.