What a funny thing to ask. It's actually up to the breeder's or trainer's preferences. For example you need a 0 IV on the S. Attack area if your Pokémon is a pure Physical attacker, in that case you should get a Pokémon with the Adamant nature. [Mega] Mawile is the best example for this.
An another example is when you need to make use of a Pokémon's disadvantage as your advantage. A Quiet [Mega] Ampharos is prefered over a usually seen as perfect Modest one, while that sounds funny or weird, but that is true -- to make sure the Mega Ampharos gets hit first when switching out with the move Volt Switch. That said quiet Ampharos needs a 0 IV Speed AND 0 IV on Attack as well! The reason for Speed has been explained well, while the reasoning for the attack is actually obvious as well -- Attack department is not needed for a Special based attacked! Incidentally that Quiet Ampharos pairs perfectly with a Glass Cannon, for example like Gengar, Breloom, Roserade etc. But that doesn't mean an Ampharos with a Modest nature is a third rate, it is just as useful as a Quiet one. It's their role that differs.
So you can consider a Pokémon's role and if it sync perfectly/greatly with its team mate. EV is suggested to be planned beforehand on breeding as well. :)
Hope that helps Mr. Donut. ;)