Do Not Ban
Porygon-Z has yet to demonstrate to me that it is appreciably powerful enough to be seriously considered to be banned. While it is a powerful Pokemon that can beat a number of teams right now, I believe this is due to it taking advantage of the primarily physical bias in the current metagame, as opposed to it being too strong for it. In the form of checks and counters to Porygon-Z, both offensive and defensive teams have viable options for defeating it. On stall, Slowking and Munchlax are both able to switch in and beat Porygon-Z; Slowking can paralyze it, crippling Porygon-Z, while Munchlax can sponge its attacks all day, and phaze it if it decides to boost. For offensive teams there is no shortage of Pokemon that are capable of outspeeding it and either killing it outright or putting it in range of a revenge kill, due to Porygon-Z's base 90 speed stat, which while by no means is poor, is hardly the speed necessary for an easy sweep.
I also do not believe that any of Porygon's individual sets are inherently broken, due to the fact that Porygon cannot OHKO or outspeed many Pokemon without a boost, and the fact that Porygon is not sufficiently bulky to easily obtain such a boost. Naturally these boosts can be obtained through use of items such as a Choice Scarf or Choice Specs, but the inherent costs of such boosts, the locking in of a move, negate the benefits supplied in my opinion.
To touch on a final point, much has been made of the fact that Porygon-Z can 1 or 2 HKO the entire game without fail with the assistance of a full Spikes complement. While this is certainly an impressive statistic, I do not believe that it is a viable reason to ban Porygon-Z, one because laying 3 layers of Spikes is not the easiest thing in the world, particularly when you aren't running a stall team, and two, because frankly spinning in this metagame is not difficult, with effective spinners such as Cryogonal and Kabutops that are able to threaten spinblockers, not to mention Hitmonchan who is capable of running Foresight to negate the ability of the Ghost to spinblock at all. Therefore I do not feel that hazard support is a viable argument to say that Porygon-Z is broken.
In summary, I do not believe Porygon-Z is broken in RU due to my opinion that it is not breaking the metagame, simply taking advantage of specially defensive deficiencies. That being said, with the future metagame shift upcoming, with Qwilfish and especially Krookodile leaving the tier, it is possible that Porygon-Z may prove itself to be too powerful at that time, thus I believe that a future suspect discussion on Porygon-Z may be necessary at a future suspect period.