Chansey
A- to B+ Pretty much has been stated by other. Passive as hell in a meta game where passiveness is not taken kindly, too many new relevant threats that completely don't care or screw it over. It's still a good mon in terms of clerics but it's not as useful anymore and I prefer to have my wish passers/clerics now to have some decent offensive presence or a better typing to handle the relevant stuff.
Skarmory
A- to B+ Its limitations in terms of what it does on a consistent basis is less appreciated due to how the meta game is now. Magnezone's relevance now is a big problem for Skarmory when a lot of teams now run it with everything Skarmory would've normally checked before more consistently. Too much offense for Skarmory to actually keep up. B+ is fine.
Slowbro
A- to A This thing is good, and I mean really good. It's only true limitation is its special defense and only a handful of wall breakers or specially strong mons such as M-Heracross, M-Gardevoi, Thundurus, the usual shit. After that it's just a bunch of useful pros. Versatility in terms of movepool of what it chooses to handle more efficiently, a nice sexy defense stat, Regenerator, some decent offensive presence, great glue in defensive or balanced cores, etc. If it wasn't for everyone spamming the same old op stuff I would see this up in A+, but unfortunately that isn't the case, A is definitely good.
Manaphy
Keep at B+ I love Manaphy but I feel the metagame is too offensive for it to warrant a move up to A-. Needs a bit of support in a similar way to all the B+ mons but still solid. Haven't been able to test out TailDance though so I'm not necessarily sure if that's enough to push it to A-.
M-Alakazam
Whatever The Holy Ranking Council Wants I've used this a lot and I think it's really kickass, I still think it's a B+ mon but w/e. We give out opinions, like 4 or so people use those to add on to their own logic, and ta da you have your rankings and then the circle of VR thread life continues. *Insert Circle Of Life song here*.
M-Aerodactyl
B+ I've only seen a couple of good people use this well and that was more based on their own skill level than M-Aeros actual viability. I don't think it's viable at the level of A- personally, it just seems like people want it to move up cause it has a couple useful tools when it practices it kind of just dies and is overrated. That's just me though.
Cresselia
B- Cresselia is underrated. Walls a lot of of stuff consistently like M-Cham, has a nice move pool to back it up, not bad at all and better than half the stuff in B-. I'll be honest though I think the B rank is a bit of a mess in the mid to lower area. I don't really know how you guys,
alexwolf included, go about deciding what the discussion should be when you do your updates in terms of order. I would like to see some discussion on Scizor and some of the mid B rank stuff moving up or down, cause I think regular Scizor is a waste of team slot imo. Not now though just next update or w/e. Threads already clogged up with people wanting things like Lando-I to move down (lol) and just random stuff that makes no sense so I rather just wait till then to discuss it.
Thundurus-T
C Thundurus Therian maintained it's C+ rank during a time where stall was a more relevant style of play in OU, in terms of consistency and what people generally used as a playstyle. Unfortunately, that time has passed and Thundy-Ts main niche as a stallbreaker, which I think it's all it's generally good for, is less appreciated these days. At this point Thundy-T on more serious team builds is generally an option similar to those if the C ranks, where they have some useful roles on a team, but require a generally decent amount of synergy and support to truly function.
Jellicent
Unranked Jellicent is barely usable honestly, slow taunts are just dumb imo when they'll get the support move before you get the chance to taunt them, Will-O-Wisp is done by a lot better stuff like Sableye, doesn't really wall anything besides the obvious like Keldeo which shouldn't warrant a rank honestly. Spin blocker I guess but its roles are so niche individually and combined that there's not a huge point of running this unless you are desperate for an answer to keldeo, considering we got stuff like celebi and AV Azu you really shouldn't though.
Klefki
C+ I had a lot of extensive use with this one on a HO team utilizing dual screen in the past with M-Ala and some other stuff. With that being said here's why I think it should stay in C+. It's outclassed by screens Azelf, a B- mon. Yes before somebody goes off and goes "But dude Prankster" that's great and all, but running Klefki means I have to use 2 mons to get Rocks up as well. It can't stall break, what so ever. The lack of Taunt is just crippling because it becomes free turns for a lot of stuff. Also I think spikes are only effective against stuff like balanced teams that have too many grounded mons. The issues with spikes in OU is that so many relevant threats in the metagame have no care for it, the Latis being one of the most used offensive defoggers doesn't help either. I see it as something like Shuckle in the C+ area where it has a good support role for its specific role, but that's really what it's limited to which isn't necessarily a bad thing, it's just not great.