Why this belongs on-site
Porygon-Z is a powerful Pokemon and is a great choice user. Although it has counters, those can be gotten around by using certain coverage moves. It has viability over similar Pokemon because of its raw power, boosting moves and single weakness.
Porygon-Z
[Overview]
<p>Will write..</p>
[SET]
name: Agility
move 1: Agility
move 2: Tri-Attack
move 3: Dark Pulse / Ice Beam
move 4: Hidden Power Fighting / Thunderbolt
ability: Download
item: Life Orb
nature: Modest
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Speed boost makes him very capable of a sweep with his great SpA and Adaptability
-Tri Attack is a general STAB move
-Agility for more Speed allowing a sweep
-Dark Pulse and HP Fighting for better coverage, hitting most threats
-Use Ice Beam and Thunderbolt together for BoltBeam coverage
-Download's hopeful +1 boost will make Porygon-Z even more strongth allowing a fast and powerful sweep
[ADDITIONAL COMMENTS]
-HAZARD SUPPORT
-Timid is viable for more Speed, although a lot less powerful
-Psychic can hit Terrakion much harder
-Psyshock can hit the physical side of the spectrum
-Hidden Power Fire can be used to hit Scizor and the like
-Leftovers can be used to take off residual damage
-Silk Scarf makes your Tri Attack more powerful
-Expert Belt grants you additional power when hitting things like Dragonite
-Adaptability can be used, although Porygon-Z will miss Download's great power boost
-Gengar is again a good partner
-Wobbuffet can really help against a lot of things that annoy P-Z
-Dual Screeners like Azelf, Deo-D, and Latias can really help P-Z's defenses, letting him boost
-Stuff that slows down VoltTurn like Forretress, Rotom-W and Scizor make its job easier
-The two blobs wall this, and Ferrothorn can give a lot of trouble if Porygon-Z lacks Hidden Power Fighting
-Things that stop priority are heaven-sent (Magnezone stops Scizor, Skarm against Dnite, Gliscor against Conk, etc.)
[SET]
name: Nasty Plot
move 1: Tri Attack
move 2: Nasty Plot
move 3: Thunderbolt / Dark Pulse
move 4: Hidden Power Fighting / Ice Beam
ability: Adaptability
item: Life Orb / Chople Berry
nature: Timid
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Nasty Plot not outclassed by Mew and Celebi because of the high SpA and powerful STAB
-More of a wall breaker than Agility
-Tri Attack is great, powerful STAB with Adapability
-HP Fighting + Thunderbolt with Nasty Plot allow all Steel-, Rock-, and Ghost-types to be OHKO's at +2 SpA
-BoltBeam can be used too
-So can Dark Pulse + Hidden Power Fighting
-Life Orb gives great power, Chople Berry is to help against faster Fighting-types
[ADDITIONAL COMMENTS]
-Not many other options are recommended outside of analysis
-Fast Fighting-types can easily stop Nasty Plot, while priority can put a big dent in too
-CB Terrakion is pretty much a hard counter
-Dnite can stop it with Multiscale
-Use hazards for that reason exactly!
-Nothing can really tank a hit from this if it gets to +2 making switching in hard
-Gengar is a good partner because Porygon-Z softens up walls for it
[SET]
name: Choice
move 1: Tri Attack
move 2: Ice Beam
move 3: Hidden Power Fire / Hidden Power Ground
move 4: Trick
ability: Adaptability
item: Choice Scarf / Choice Specs
nature: Modest
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Powerful scarfer
-Good for revenging weakened threats
-One of the most powerful Specs users in the game
-Both have a hit-and-run playstyle
-Tri Attack is the main move, 160 BP move with Adapability is no joke
-Ice Beam is good for picking off Dragons, Landorus and Gliscor
-Hidden Powers both have their advantages, Fire hits Bug or Grass/Steel and Ground can hit Heatran hard
-Trick is great for screwing with walls like Blissey and Chansey
[ADDITIONAL COMMENTS]
-EVs are obvious
-Psychic / Psyshock are both viable to hit Fighting-types and Gengar
-Thunder on a rain team
-Don't use Hyper Beam
-Adaptability is recommended for powering up Tri Attack, but Download is usable, getting a much larger boost than Adaptability
-Don't try to take hits, but he can actually survive a CB Scizor Bullet Punch after rocks
-Gengar works well with Porygon-Z, immune to Fighting while Porygon-Z is immune to Ghost. Gengar can also handle Fighting types that plague Porygon-Z.
-Terrakion, Blissey, and Chansey all wall this, although Terrakion can be nailed by Psychic
[Other Options]
-Choice Specs set with Tri-Attack, Adapability and cover is insanely strong
-Scarf set with Download is a decent revenger
-Nasty Plot has power, but is too slow
-Porygon-Z has access to Trick Room, so a minimum Speed set with three attacks and Trick room can be used, although things can easily "outslow" this
-Recover can be used with a set up move or three attacks, but Porygon-Z is frail and wants to focus on attacking
-Don't use Hyper Beam
-Substitute can be used with three attacks, but usually Porygon-Z is better just firing off Tri Attacks
-Shadow Ball and Signal Beam can be used for coverage on any set
-Thunder on rain teams, Blizzard on hail teams
[Checks and Counters]
-Ferrothorn can act as a hard stop when not carrying Hidden Power Fighting or Fire
-Chansey and Blissey can take anything Porygon-Z throws at them
-Jirachi, Heatran, and Magnezone all resist or take very little damage from Porygon-Z that lack HP Fighting
-Although Scizor's Bullet Punch can't OHKO, with residual damage Porygon-Z will perish shortly
-Terrakion can outspeed non-Scarf and a boosting set that lacks Agility and promptly OHKO with Close Combat
-Jellicent can take an unboosted Thunderbolt or Dark Pulse, but has trouble with Nasty Plot and Specs variants
-Conkeldurr has enough bulk to take unboosted attacks and return with a strong Drain Punch
Porygon-Z is a powerful Pokemon and is a great choice user. Although it has counters, those can be gotten around by using certain coverage moves. It has viability over similar Pokemon because of its raw power, boosting moves and single weakness.

Porygon-Z
[Overview]
<p>Will write..</p>
[SET]
name: Agility
move 1: Agility
move 2: Tri-Attack
move 3: Dark Pulse / Ice Beam
move 4: Hidden Power Fighting / Thunderbolt
ability: Download
item: Life Orb
nature: Modest
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Speed boost makes him very capable of a sweep with his great SpA and Adaptability
-Tri Attack is a general STAB move
-Agility for more Speed allowing a sweep
-Dark Pulse and HP Fighting for better coverage, hitting most threats
-Use Ice Beam and Thunderbolt together for BoltBeam coverage
-Download's hopeful +1 boost will make Porygon-Z even more strongth allowing a fast and powerful sweep
[ADDITIONAL COMMENTS]
-HAZARD SUPPORT
-Timid is viable for more Speed, although a lot less powerful
-Psychic can hit Terrakion much harder
-Psyshock can hit the physical side of the spectrum
-Hidden Power Fire can be used to hit Scizor and the like
-Leftovers can be used to take off residual damage
-Silk Scarf makes your Tri Attack more powerful
-Expert Belt grants you additional power when hitting things like Dragonite
-Adaptability can be used, although Porygon-Z will miss Download's great power boost
-Gengar is again a good partner
-Wobbuffet can really help against a lot of things that annoy P-Z
-Dual Screeners like Azelf, Deo-D, and Latias can really help P-Z's defenses, letting him boost
-Stuff that slows down VoltTurn like Forretress, Rotom-W and Scizor make its job easier
-The two blobs wall this, and Ferrothorn can give a lot of trouble if Porygon-Z lacks Hidden Power Fighting
-Things that stop priority are heaven-sent (Magnezone stops Scizor, Skarm against Dnite, Gliscor against Conk, etc.)
name: Nasty Plot
move 1: Tri Attack
move 2: Nasty Plot
move 3: Thunderbolt / Dark Pulse
move 4: Hidden Power Fighting / Ice Beam
ability: Adaptability
item: Life Orb / Chople Berry
nature: Timid
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Nasty Plot not outclassed by Mew and Celebi because of the high SpA and powerful STAB
-More of a wall breaker than Agility
-Tri Attack is great, powerful STAB with Adapability
-HP Fighting + Thunderbolt with Nasty Plot allow all Steel-, Rock-, and Ghost-types to be OHKO's at +2 SpA
-BoltBeam can be used too
-So can Dark Pulse + Hidden Power Fighting
-Life Orb gives great power, Chople Berry is to help against faster Fighting-types
[ADDITIONAL COMMENTS]
-Not many other options are recommended outside of analysis
-Fast Fighting-types can easily stop Nasty Plot, while priority can put a big dent in too
-CB Terrakion is pretty much a hard counter
-Dnite can stop it with Multiscale
-Use hazards for that reason exactly!
-Nothing can really tank a hit from this if it gets to +2 making switching in hard
-Gengar is a good partner because Porygon-Z softens up walls for it
name: Choice
move 1: Tri Attack
move 2: Ice Beam
move 3: Hidden Power Fire / Hidden Power Ground
move 4: Trick
ability: Adaptability
item: Choice Scarf / Choice Specs
nature: Modest
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
-Powerful scarfer
-Good for revenging weakened threats
-One of the most powerful Specs users in the game
-Both have a hit-and-run playstyle
-Tri Attack is the main move, 160 BP move with Adapability is no joke
-Ice Beam is good for picking off Dragons, Landorus and Gliscor
-Hidden Powers both have their advantages, Fire hits Bug or Grass/Steel and Ground can hit Heatran hard
-Trick is great for screwing with walls like Blissey and Chansey
[ADDITIONAL COMMENTS]
-EVs are obvious
-Psychic / Psyshock are both viable to hit Fighting-types and Gengar
-Thunder on a rain team
-Don't use Hyper Beam
-Adaptability is recommended for powering up Tri Attack, but Download is usable, getting a much larger boost than Adaptability
-Don't try to take hits, but he can actually survive a CB Scizor Bullet Punch after rocks
-Gengar works well with Porygon-Z, immune to Fighting while Porygon-Z is immune to Ghost. Gengar can also handle Fighting types that plague Porygon-Z.
-Terrakion, Blissey, and Chansey all wall this, although Terrakion can be nailed by Psychic
[Other Options]
-Choice Specs set with Tri-Attack, Adapability and cover is insanely strong
-Scarf set with Download is a decent revenger
-Nasty Plot has power, but is too slow
-Porygon-Z has access to Trick Room, so a minimum Speed set with three attacks and Trick room can be used, although things can easily "outslow" this
-Recover can be used with a set up move or three attacks, but Porygon-Z is frail and wants to focus on attacking
-Don't use Hyper Beam
-Substitute can be used with three attacks, but usually Porygon-Z is better just firing off Tri Attacks
-Shadow Ball and Signal Beam can be used for coverage on any set
-Thunder on rain teams, Blizzard on hail teams
[Checks and Counters]
-Ferrothorn can act as a hard stop when not carrying Hidden Power Fighting or Fire
-Chansey and Blissey can take anything Porygon-Z throws at them
-Jirachi, Heatran, and Magnezone all resist or take very little damage from Porygon-Z that lack HP Fighting
-Although Scizor's Bullet Punch can't OHKO, with residual damage Porygon-Z will perish shortly
-Terrakion can outspeed non-Scarf and a boosting set that lacks Agility and promptly OHKO with Close Combat
-Jellicent can take an unboosted Thunderbolt or Dark Pulse, but has trouble with Nasty Plot and Specs variants
-Conkeldurr has enough bulk to take unboosted attacks and return with a strong Drain Punch
Side notes:
In reference to this post, I'd like to point a few things out.
The reason Porygon-Z was declared uncompetitive was because it was outclassed by other Pokemon, including Nasty Plot users like those above. Since this was posted over a year ago, Thundurus has been banned. In addition, Mew and Celebi lack some qualities Porygon-Z has. Mew simply lacks the strength to break through walls Porygon-Z can, and lacks the powerful STAB move he has too. Celebi doesn't have the coverage or STAB power Porygon-Z has.
This is also true for several choice bearing Pokemon as well. Gliscor can abuse Choice Terrakion, but it is still common. Choice Rotom-W is stopped hard by Ferrothorn and Gastrodon, but it still sees use. Scarf MoxieMence is stopped when locked into and Outrage, as is Choice Band Dragonite. These are just risks of using Choiced Pokemon. Prediction is a key.
The former is rarely seen in OU, while the latter is easily taken down by a Hidden Power Fire. CB Scizor's Bullet Punch fails to OHKO Porygon-Z, and that stands for weaker priority as well.
I know I'm gonna get hell for this ;-;
In reference to this post, I'd like to point a few things out.
Porygon-Z can run Nasty Plot, but why would I use it when I could use other / better Nasty Plot users such as:
Thundurus
Has Mischievous Heart, is much faster, is resistant to Mach Punch and Bullet Punch, meaning it isn't revenge killed as easily as Porygon-Z is.
Mew
Also has Nasty Plot and is significantly more bulky than Porygon-Z / has better typing. Unlike Porygon-Z, it has access to Flamethrower, allowing it to torch Steels. Porygon-Z has to rely on HP Fighting to beat Tyranitar, while Mew has Aura Sphere. Lastly, mew gains STAB on Psycho Shock unlike Porygon-Z, making it easier for it bypass Blissey.
Celebi
Has more bulk and resistances, making it easier to set-up. Also has Giga Drain + Recover to keep it healthy (Porygon-Z has Recover, but it's too frail to use it and loses out on coverage).
The reason Porygon-Z was declared uncompetitive was because it was outclassed by other Pokemon, including Nasty Plot users like those above. Since this was posted over a year ago, Thundurus has been banned. In addition, Mew and Celebi lack some qualities Porygon-Z has. Mew simply lacks the strength to break through walls Porygon-Z can, and lacks the powerful STAB move he has too. Celebi doesn't have the coverage or STAB power Porygon-Z has.
Choice sets are ridiculously easy to counter; tons of things can switch-in on Tri Attack, such as Tyranitar, Jellicent, Ferrothorn, Terakion, Blissey, Chansey, and so on. All of these Pokemon are common and can easily take advatnage of a Porygon-Z locked into a single move.
This is also true for several choice bearing Pokemon as well. Gliscor can abuse Choice Terrakion, but it is still common. Choice Rotom-W is stopped hard by Ferrothorn and Gastrodon, but it still sees use. Scarf MoxieMence is stopped when locked into and Outrage, as is Choice Band Dragonite. These are just risks of using Choiced Pokemon. Prediction is a key.
Agility sets are too easily revenge killed by Roobushin and Scizor.
The former is rarely seen in OU, while the latter is easily taken down by a Hidden Power Fire. CB Scizor's Bullet Punch fails to OHKO Porygon-Z, and that stands for weaker priority as well.
I know I'm gonna get hell for this ;-;