I generally gear up my mindset based on ladder differentials, let's assume that I have a Magic Coat Smeargle in against a Darkrai (just accept the scenario I have strange teams :) ).
If I see a +3, -20 then I will Magic Coat, because odds are that the player will just make the normal move and Dark Void.
However, If I see a +20, -3 then I will expect them
to expect funny business. I will probably just Spore as they Dark Pulse and bring me down to my sash.
Ladder Differentials should always be your strategy guide when no other information is available about your opponent, until you have information that indicates they are on some next level shit, IE double switching.
This of course all changes after I have played someone at least once and we remember each other's moves and style. At that point, I agree with Poppy's quote:
Originally Posted by Fat Poppy
Not necessarily, there's normally a 'best' move, or one that can't screw you over, even if you do mispredict, but rewards you if you don't.
as the best guide; best in this case meaning statistically most likely to secure the most wins.