I would Say that you should go with 252 Special Attack/128 Defense/128 Special Defense and the rest on HP because the more defense/special defense you have, the more boost you have from Eviolite.
EDIT - I would think twice before Maxing Special attack - with 252 on both defenses and 4 on HP you can wall pretty much everything, and the Quiet-31 IVs boost would work enough. After all, Porygon2 with Eviolite is a tank, and he does have a decent special attack, it will deal damage nicely with Adaptability Stab moves (I'd suggest to pair with Conversion and Bolt-Beam)