Porygon-Z: Do Not Ban
I've thought about this for the last couple of days, and chances are when something is broken in the metagame I can tell pretty quickly. I don't have that feeling about Porygon-Z who has been surprisingly underwhelming in my experience. The special attack is definitely amazing with download backing it, but it's main attack (Tri Attack) is coming off of a pretty bland attacking type for RU. Most teams have a Ghost type as their spin blocker, which is an automatic check for the Scarf set and many teams have a Rock type or Steel type as well, preventing it from a late game clean up.
Porygon-Z is also quite frail and not overly fast. This hinders any of the sweeper sets from completely dominating and makes it not more threatening than any of the other special sweepers (Moltres, Rotom, Sceptile, etc). Priority is extremely prevalent in RU, with must teams having 1-3 priority users really prevent Porygon-Z from realising its true potential.
To summarize, Porygon-Z, while a very good pokemon in most respects, just falls short of being broken because of its sub-par attacking type, frail defenses and weakness to priority.