Since I've done two, I'll take on
Mega Manectric and
Terrakion, along with
Chansey.
On the subject of
Mega Manectric, I'm nominating it go from
B ---> B+ / A-.
Mega Manectric is blazing fast with base 135 Speed, it also is only ~4% weaker than Life Orb Manectric is. If I did this correctly, it should have around 130 Defense after Intimidate:
-1 0 Atk Mew Zen Headbutt vs. 0 HP / 0 Def Mega Manectric: 69-82 (24.5 - 29.1%) -- 99.8% chance to 4HKO
0 Atk Mew Zen Headbutt vs. 0 HP / 0 Def 130 Base Defense Mega Manectric: 69-82 (24.5 - 29.1%) -- 99.8% chance to 4HKO
So, correct me if I'm wrong, but on any Pokemon it switches into, Mega Manectric instantly gains a stat spread of 70 / 75 / 130 / 135 / 80 / 135. It has an extremely powerful Thunderbolt and also has access to many interesting coverage options such as Hidden Power [Ice] and Overheat. It makes an exceptional pivot with Volt Switch + Intimidate and blends perfectly with Landorus-T forming an infamous core. Mega Manectric is a very good Pokemon, however, the problem with it is that it is always left being walled by something, it's always walled by Mega Venusaur, Tyranitar, and Chansey, while if it chooses Hidden Power [Ice] it is walled by Quagsire and Latias, while if it chooses Hidden Power [Grass] it is walled by Kyurem-B, Garchomp, and Dragonite. I can keep going on with things that wall Mega Manectric, but that's no fun! Another issue is you're stuck with 105 Speed until you Mega Evolve, which can be very problematic. On the bright side, if you switch into a Electric-move, Mega Manectric becomes insanely powerful! Mega Manectric's coverage is quite amazing though and with proper team support it can plow through offensive teams and slower teams, even fast ones too, because hell, it is faster than most! For the reasons of a great blend of power and pseudo-bulk, I believe that Mega Manectric belongs in either B+ or S-.
Terrakion
Terrakion, the infamous swordsman, makes a return to OU this generation! Terrakion is a phenomenal Pokemon that can perform many different roles from Lead, to Swords Dance, to Choice Scarf. Terrakion has a sky-high Attack stat of 129, backed by a base Speed of 108 that allow it to outspeed the majority of the tier. Another amazing feature of Terrakion is its STAB moves which are unresisted by any common OU Pokemon bar Mega Medicham and Aegislash, who is taken care of thanks to Earthquake. Terrakion has very few counters, the best being Landorus-T, Gliscor, and Slowbro, the former two being handled by Hidden Power [Ice] on lure sets. Finally, Knock Off just made Terrakion better as if it switches in on a Knock Off it gets a +1 thanks to Justified. Terrakion is truly deserving of its title of a 'Legendary Pokemon'!
Chansey
Gary2346