Defence and stamina IVs matter more when the species have less of them in their base stats, this means more contribution to the overall max CP. But does that really mean that the other attribute that the mon has a lot of is inferior? A mon with 200 defence and 150 stamina, will defence really matter less or will it just be less max CP?
Tankiness is def*hp, so the IV of the lower base stat does actually matter more. You always get a bigger product if you raise the smaller of the two coefficients rather than the larger.
EDIT: To illustrate this, (150+15)*200 = 33000, 150*(200+15)= 32250.
The truth isn't that simple however, because breakpoints and bulkpoints.
^This!
Should you stumble across a 100% IV mon in your travels, favourite it, love it, tell all your friends about it!
However, it makes the experience further enjoyable if you're capable of settling for the best that RNG is willing to provide you on any given day. RNG is a very moody fellow, which we all must learn to live with.
Of course, if a type has really high (say) HP base stats, then the added HP for your specific Pokémon would be somewhat less important than the Attack, or Defensive stat for that mon...in my opinion.
Everyone always says that the added Attack stat is most important, but really, why would that matter if its deficient in the other two? It would just faint earlier and not get to benefit from its high Attack stat then!
So *I* think that the best IVs are those that are somewhat evenly distributed across all three. Though mons with low base stats for HP, but high base stats for Attack (like Gengar) I'd want high HP IVs for him.
