Well the ones that are necessary are Attack, Special Attack and Speed, and Special Defense is probably the least important because of your naive nature.
It would be best though, to have six IVs, if you are willing to work for it.
So IVs you should definitely have are HP, Attack, Defense, Special Attack and Speed
Now here are all the ways to boost happiness in X and Y:
Sorry the screenshot is terrible, but I can't edit it(now).