Did you once saw a pokemon which you though it should have a move but it doesn't have it so it can be much better competitively JUST one move to make the difference between bad and good.

Like we all say "Give flareon Flare Blitz!"

so which move (that exists...) can make a bad pokemon a really good one?

Also don't mention quiver dance on stuff like gardevoir cause gardevoir is far from being a butterfly, something that they can use somehow.