Thread has been kind of bad so...
Ferrothorn could possibly go to A. Based on the moves it runs it can be setup bait for Charizard X, Clefable, nearly anything is possible because it has pretty bad 4MSS. The only problem with it dropping is that it handles sand and rain offense really well outside of some rarer Pokemon, walling Kingdra, Low Kick-less Kabutops, any form of Politoed, Excadrill, among others. The only real flaws are 4MSS, that Fire weakness which leaves it as Clef and Zard bait, and it being prone to being worn down thanks to no recovery. I think its 4MSS, problems with common Pokemon like Clef, Zard and Taunt Thundurus don't really outweigh handling top threats like Azu, Drill, HP Fire-less Latis. Also, it isn't immediately OHKOd by a weak HP Fire. Some even fail to 2HKO, especially after Leech Seed and Protect. It's also immune to Toxic and if running Gyro Ball doesn't mind paralysis and is immune to Spore/Sleep Powder. I think its access to hazards is also being underestimated too, this is what sets it apart from walls like Slowbro more than anything, along with its great typing. Keep it A+ or put it in A, either is fine, but just wanted to bring that up.
Pinsir (Mega) This thing is not going to S. Everything that outspends it in OU can do a ton of damage, and that Rocks weakness. It relies on Quick Attack a lot if it wants to sweep, and if it wants to come in often it needs Defog. It has decent synergy with the Latis, but Defog isn't exactly mandatory on Latios. Keep A+
Venusaur (Mega) Sand is getting better, and while its offensive set is good, it loses out on coverage or Sleep Powder, and since sand really i that good it often ends up with only 19% recovery. Toxic immunity is nice but I don't think it's that good, but I know others disagree, I personally don't like it that much. Consider dropping it to A but I doubt it will happen
Gardevoir (Mega) I think this is possible to join A+. It's just so strong. The Speed more than makes up for the power difference with Specs Sylveon. It doesn't have much sweeping potential and it's outclassed there by Clefable, but its movepool sets it over the edge. Hyper Voice is the only truly mandatory move. Psyshock, Focus Blast, Will-O, Taunt, HP Ground, Shadow Ball, even Destiny Bond all work in the other slots.
Not nomming it at all, but give Mega Tyranitar some more credit. If you find it walled by Ferro, be smart and run Fire Punch over EQ.
Hippowdon should stay A-. It's a great wall with good typing, and is often seen as a more passive TTar when it has an entirely different role.
Jirachi This needs to join A- for reasons mentioned (though not really well) It has lost a lot in the ten shift, but its typing is still good. It handles Mega Gardevoir, checks Heracross if it runs Zen Headbutt, and is just in general a great mon that even gets Rocks, Wish, Healing Wish, a great SubToxic set, possible SubCM sets, Scarf, mixed, physical and others. It's better than Mandibuzz for sure, don't even compare it to that
Magnezone A-, for reasons already said
Starmie A- should be reconsidered, it's just such a great offensive mon in general thanks to Analytic and is the best dedicated spinner in OU, and it can work on offense with a LO set, and has a decent defensive spin set too. I can't believe both this and Rachi were once unranked.
Victini I'm not entirely sure it deserves A-, it has a lot of significant flaws such as being Pursuit or trapping bait after a V-create, I say keep B+
Celebi B+ is perfectly reasonable for a mon with recovery, Heal Bell, U-turn/BP to escape Pursuit, Nasty Plot, a great SpDef set, making it a Keldeo counter, and of course PerishTrap :]
Staraptor I've gone over this twice so look at older posts of mine, but bump Raptor to B+
Quagsire It's not that good anymore, B-
Weavile Already said why but B
Zapdos I disagree with it dropping because you're all using it as a Birdspam counter when it's not. It's a Defogger that check Pinsir and can pivot with Volt Switch
Aggron (Mega) It has the most physical defense in the tier and an amazing typing. If you hate its lack of recovery hat much, Fairy-type Wish passers have good synergy with it. Keep it in C+, it's not Bronzong level garbage
Crawdaunt It should go up not down... this thing has no realistic switch ins and has priority to fix its Speed, and it isn't
ridiculously frail
Espeon Why is this C+ ???
Klefki I said why before but B-
Gastrodon Drop to C please, it's not good.
Porygon2 Once again, C+
Tornadus I made a post about Tornadus a while back but never officially nonmed it. Find that post for reasoning, C.
Blissey Get out and blacklist please
Zygarde C- for Zygarde and Mence while I'm at it for reasons mentioned by a ton of people...
Ferro: Possibly A
Pinsir: Stay A+
Venusaur: Possibly A
Gardevoir: Possibly A+
Run Fire Punch Mega TTar people
Hippowdon: Stay A-
Jirachi: A-
Magnezone: A-
Starmie: A-
Victini: Stay B+
Celebi: B+
Staraptor: B+
Quagsire: B-
Weavile: B
Zapdos: Stay B-
Aggron: Stay C+
Crawdaunt: B-
Espeon: C
Klefki: B-
Gastrodon: C
Porygon2: C+
Tornadus: C
Blissey: Remove and blacklist
Zygarde: C-
Salamence: C-