This core I've been trying, with moderate success:
252 HP, 252 SpDef, 4 SpA
252 HP, 252 Def, 4 Atk
Porygon2 is primarily a special wall, stopping threats like Zapdos and Chandelure in their tracks. Trace is a very good ability that lets me beat a variety of threats. Recover is great, reliable recovery, and Tri Attack is strong STAB. BoltBeam for coverage. I run Thunderbolt to compensate for the loss of power caused by not running Download or Analytic. Qwilfish's main job is to counter the Fighting-types that give Porygon2 so much trouble. It's great typing, ability, and support movepool make it very good. Waterfall is strong STAB, Pain Split for recovery (only one besides Rest), Thunder Wave to spread status, and Spikes for hazard damage. I've also considered Haze, but I prefer Spikes.