Porygon Z might soon be more attainable in Pokemon GO if Porygon is chosen for this month's Community Day. Porygon Z is the third and final evolution of Porygon. In the main series games, it has traditionally had one of the highest special attacks of any non-legendary Pokemon.
Honestly considering the trouble you have to go through to get it, evolution via trade using two different items, you'd hope it would have impressive stats. In Pokemon GO It is also somewhat difficult to attain, but It's high attack power can make it worth it.
Porygon Z Pokemon GO: How to Catch
The largest barrier you'll have to overcome to catch Porygon Z is Porygon2. To evolve Porygon into Porygon2, much like in the main series games you need an upgrade, these can be found as research rewards or occasionally from Pokestops at high streak levels.
You also will need 25 Porygon Candies. Once you've managed to get a Porygon2, to get Porygon Z you need 100 Porygon Candies and a Sinnoh Stone. Sinnoh Stones can be found as research rewards or in the aftermath of trainer battles.