So people are already talking about porygon2! I came here to check on its ranking in viability and I agree, it deserves higher ranking. As a wall, it can wall the most of the ou metagame, and then toxic stall as well. It is weak to fighting, so should be teamed up with a check to things like conkeldurr, terrakion, and breloom. A few pokemon who can boost their atk or spatk can take advantage of porygon2. It is not immune to toxic. It needs some support. Doesn't this describe the 'A' rank?
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time.
Let me run some damage calculations to show its viability.
252 SpA Keldeo Secret Sword vs. 252 HP / 184+ Def Eviolite Porygon2: 152-182 (40.64 - 48.66%)
252 SpA Life Orb Sheer Force Landorus Earth Power vs. 252 HP / 72 SpD Eviolite Porygon2: 148-175 (39.57 - 46.79%)
+1 252 Atk Genesect U-turn vs. 252 HP / 184+ Def Eviolite Porygon2: 90-106 (24.06 - 28.34%)
252 SpA Choice Specs Latios Draco Meteor vs. 252 HP / 72 SpD Eviolite Porygon2: 220-261 (58.82 - 69.78%)
-2 252 SpA Choice Specs Latios Draco Meteor vs. 252 HP / 72 SpD Eviolite Porygon2: 111-132 (29.67 - 35.29%) (cannot be 2hkod)
252+ Atk Tyranitar Superpower vs. 252 HP / 184+ Def Eviolite Porygon2: 162-192 (43.31 - 51.33%)
252 Atk Life Orb Mamoswine Superpower vs. 252 HP / 184+ Def Eviolite Porygon2: 187-221 (50 - 59.09%)
252 Atk Garchomp Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 108-127 (28.87 - 33.95%)
252 Atk Salamence Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 111-132 (29.67 - 35.29%)
0 SpA Porygon2 Ice Beam vs. 0 HP / 0- SpD Salamence: 384-452 (116.01 - 136.55%) -- guaranteed OHKO
252 Atk Kyurem-B Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 132-156 (35.29 - 41.71%) -- guaranteed 3HKO
That's all without abusing its ability.
¤ On switching in to a boost, porygon can still take out outraging dragonite thanks to its traced multiscale + bulk.
¤ Porygon2 can switch in to Jolteon's thunderbolt/volt switch to heal itself with volt absorb. It can do the same for thundurus-t, but must be wary of focus blast, which can 2hko it.
¤ If heatran's fire moves have been boosted by one of yours, that's fine, since porygon2 is immune to heatran's fire moves.
¤ He takes good use of Celebi's natural cure, being able to switch in to it, take the status, and switch out freely.
¤ Porygon2 can use Dugtrio's arena trap against it, trapping it in for an easy kill with toxic/ice beam.
¤ Perfect check to gyarados, especially if gyarados runs intimidate.
¤ Same goes for landorus-t
¤ It can switch in to Jirachi for Serene Grace, and using discharge, can be very annoying.
¤ Can switch in to expected water attacks from pokemon immune to water like gastrodon with storm drain. Is immune to Jellicent's shadow ball and water attacks, though must be careful of toxic.
¤ Gliscor with toxic heal can accidentally give porygon2 toxic healing.
---Many other abilities can be traced by porygon2 for its advantage---
I think this replay may help to show some of its viability (even though I made some stupid decisions in this battle :P)
http://pokemonshowdown.com/replay/oususpecttest-34876156
You can see how I was able to wall and toxic volcarona, use trace to heal from jolteon, wall and toxic both latios and vaporeon. I could have toxic stalled them both if I wasn't impatient in this match.
While low in usage in OU and legal in UU, Porygon2 is much more viable in OU than C rank.