As you know, when you first meet the Greybeards they ask you to shout at them, to "taste of your voice." Naturally, they want you to FUS them. But there is nothing stopping you from shouting something else (except that they will keep asking until you finally do FUS them). At that point I often decide to have a little fun. Shouts I have used that I found amusing:

Ice Form: I love watching them topple over like statues.
Cyclone (Dragonborn): Weeee!

Most of the other shouts don't seem to have the same entertainment value. Fire or Frost breath just damages their health a little, but nothing much else. Can't disarm them. I have never tried Dismay because I doubt it would work.

What have you shouted at them that you found entertaining?