So, here is the next list of updates:
Mega Gardevoir: B+ ---> A-
Mew: B ---> B+
Suicune: B ---> B+
Alomomola: C+ ---> B-
Mega Aerodactyl: B- ---> B
Noivern: Unranked ---> C-
Klefki: Stays in C+
Gliscor: A- ---> A
Raikou: B- ---> B
Chesnaught: B ---> B-
Diancie: Unranked ---> C
Sylveon: B ---> B-
Crawdaunt: B- ---> C+
Mega Gardevoir: The fact that puts it a rank above Mega Medicham is its usefulness against offensive teams, thanks to a good typing and great special bulk, as well as a way more spammable STAB. Mega Gardevoir is more comparable to Kyu-B than Mega Medicham as a wallbreaker, as both Pokemon fare very well against slow teams and are decent against offensive teams too, checking some key offensive Pokemon (Latios, Latias, Keldeo, Thundurus, Mega Venusaur). Kyu-B has a ton more bulk than Mega Medicham but Mega Gardevoir is not SR weak, and both Pokemon can defeat their checks and counters with moveslot changes.
Mew: This was long overdue, Mew's Knock Off + WoW + Taunt is an amazing stallbreaker and is able to weaken whole teams on itself, while having excellent special bulk. Reliable recovery and WoW also make it able to bypass common problems of Psychic-types, such as Tyranitar, Mega Scizor, and Bisharp, which is great. There is also the Defog set, which is an excellet Defog user while retaining some of its stallbreaking qualities. Mew might even be worthy of A-, but B+ is fine for now.
Suicune: Bulky as fuck after a CM and most Pokemon struggle to even 2HKO it, while Scald fucks up many would be checks, such as Kyurem-B, Breloom, and physical Thundurus. Many teams that don't take it into account when teambuilding can easily lose against it, so it's not hard to see why Suicune is deserving of the B+ rank status for its bulky sweeping set.
Alomomola: Passes huge Wishes easily, has great mixed bulk, and never dies, while being very annoying with Toxic and Scald, Alomomola going to B- is another change that was long overdue.
Mega Aerodactyl: Excellent offensive threat with a very significant defensive presence, being able to check big offensive names such as Talonflame, Mega Pinsir, Mega Charizard X, Mega Gyarados, Mega Charizard Y, and Mega Tyranitar. Its wide coverage and access to Taunt and Roost also make it a valuable asset against defensive teams, which always have to scout for its coverage moves and a Taunt leaves them as set up bait for Mega Aero's partners. Very deserving of the B rank status.
Noivern: Meru explained it better than i can, but basically Noivern sits at an amazing Speed tier, being able to outspeed even Greninja, a huge pain in the ass for offensive teams, while packing a decent amount of punch with its high powered STABs and good coverage. Taunt, decent bulk, and decent typing also give it some switch-in chances and utility against defensive Pokemon, making it a not completely one dimensional Pokemon, such as for example Greninja. Of course it has multiple flaws, such as SR weakness, mediocre power, unreliable STABs (Hurricane is great but 70% accuracy sucks, same for Focus Blast, which is Noivern's only chance against Tyranitar and Heatran), and needing to rely on Draco Meteor to KO a lot of offensive Pokemon, which leaves it set up bait after, but C- is good enough for this fella.
Diancie: Very unsure about this, but from the comments i got from some good battlers i talked with, Diancie should be ranked somewhere in C, so i am putting it in the middle atm and we will see how it goes. I really have no experience and thus no idea about Diancie's viability guys, so don't sweat it if Diancie is misplaced, we can fix that easily in the next weeks.
Klefki: Stays in C+ rank because it lacks Stealth Rock and is somewhat passive, relying mainly on status to threaten the opponent, outside of setting up screens. Spikes are ok but nothing special in OU, especially when you lack Taunt to at least ensure they can stay up. Klefki is a decent choice in HO teams, but it's too passive to go to B-.
Gliscor: Gliscor is very comparable to Landorus-T, with Landorus-T being better offensively (great Scarf set), and Gliscor being better defensively (SubToxic, SpD with Knock Off, even SR + Taunt + U-turn). Gliscor is one of the most versatile defensive threats, able to do so many different things for a team and deal with such a wide variety of Pokemon depending on its set, it's truly worthy of the A rank. If you don't believe me, just check the WCup logs, Gliscor has been doing a hell good of a job showcasing its usefulness there.
Raikou: Everyone already explained pretty well why Raikou is on the same level with Mega Manectric, if not a bit higher, so putting it in B rank shouldn't need any explanation at this point.
Chesnaught: Chesnaught is a very situational defensive Pokemon, and it even fails to deal with one of the Pokemon it used to deal with well, Aegislash. Yeah, checking Excadrill, Bisharp, Mega Tyranitar, and Mega Gyarados is cool, but Chesnaught is not that hard to wear down, especially under sand, and struggles against most S and A+ rank threats (the entire S rank, Azumarill, Keldeo (Hidden Power Flying and Scald both suck), Greninja, Mega Pinsir, Mega Scizor, Talonflame, Fire Blast Chomp, Clefable). It has a ton of common weaknesses, lacks reliable recovery to keep up with some big offensive threats, and invites some really dangerous Pokemon such as CM Clefable, Mega Charizard Y, and Mega Charizard X to switch in for free.
Sylveon: Check Jukain's post on the same page, although he is underselling it a bit.
Crawdaunt: Check Jukain's post on the same page.
Possible rank changes to discuss:
Sylveon falling to C+
Mew rising to A-
Mega Charizard Y rising to A+
Azelf rising to B-
Celebi rising to C+
Cresselia rising to C+
Conkeldurr falling to B-
Remember though that the verdict about Aegislash's tiering is coming soon, so no update will happen until we learn what happens with Aegislash. Those are just some Pokemon you can discuss until we learn news about Aegislash, not Pokemon whose rank will change in the next few days.