Porygon-Z is gonna be one of the most OU pokemon, I just know it. Porygon 2 was good enough and they gave it an evo. With sp.att that high and choice specs = ownage.
Porygon-Z@Choice Specs
Trait: Adaptive/Download
Modest Nature
4hp/252sp.att/252speed
-Thunderbolt/Tri Attack
-Icebeam/Tri Attack
-Shadow Ball
-Hyper Beam
You can replace one of Tbolt or Icebeam with Tri Attack if your team already has members with attacks of that type. Use Adaptive if you want to add an extra "umph" to Tri Attack and Hyperbeam, otherwise use Download. Adaptive Hyperbeam even makes Blissey and Snorlax pee their pants.