I haven't seen much discussion of it here, but hands-down the best Pokemon I have used so far this period (and I've been pretty successful in a relatively short period of time) is offensive Porygon2. Most of the Porygon2 I've run into have been essentially defensive, using Trace and Thunder Wave, but that is a pretty weak version of it. I've been using Modest Porygon2 with Tri Attack, Discharge and Ice Beam--with Eviolite, you are already incredibly bulky, allowing you to switch in virtually for free on nearly all Waters, Grasses, and certain Grounds (Donphan and Hippowdon). Download is awesome, boosting your SpA to 508 if you switch in on the right Pokemon (like Hippowdon!). The best switch-ins are pretty clearly Chansey, Snorlax, and Escavalier, and thankfully those are Pokemon you should really be prepared to handle anyway.
There are other Pokemon I really think are good in this metagame that I'm not seeing much, but offensive P2 is so good that I don't want to distract from it by talking about other stuff in this post. :)