Well, a while since I've posted here. Anyways, I'll actually weigh in about these D-Rank demotions discussion
Crobat: I'd be fine with this. I haven't used Crobat, and it doesn't look superbly appealing to me. I guess it's a good Breloom check, though stall honestly isn't that common, Crobat doesn't hit extraordinarily hard, and has limited coverage. I'd use something else, and if my team needs a Breloom check, I'd use the Lati duo before this. I'm cool with moving it down to D-Rank.
Crustle: This one on the other hand I'm not so sure about. I've used Crustle quite a bit, and it's a decent choice for offensive teams to be honest. Yeah, Espeon and Xatu can bounce, but honestly you shouldn't be laying hazards if you see one of them in Team Preview. It has the nice advantage of being able to catch Espeon on the switch with X-Scissor, although it has to be paired up with a spinblocker if it wants its work to stay around. It's also pretty outclassed by Custap Skarmory. That said, I've used it to success before, so I'm honestly not sure about this.
Donphan: Nah, keep it in C. Donphan is the worst Rapid Spin user in OU by far, but it has a good place on Sun teams with its SR+Spin combo allowing it to support its team decently. It also has Ice Shard which is cool to catch Landorus-I on the switch in. It also has the bulk to check things like Terrakion. Not a great Pokemon, maybe not even good, but it should avoid D-Rank.
Durant: Hmm. I've used this thing a couple of times, and well, it's a mixed bag. It packs a ridiculous amount of power, and has a reasonable Speed tier. It also has nice coverage now with Superpower in hand, so it can pull of some sweeps here and there. That special bulk is horrible though, and things like even Latios/Latias can beat it one-on-one. High-risk, maybe high-reward. I'd be fine with D, though Low C definitely isn't unreasonable.
Nidoqueen: Well, I'd rather just split up C and put this in Low before moving it down. It's got a niche with its typing allowing it to wall Terrakion, which is pretty cool. Sheer Force also allows it to pack reasonable power, and with the movepool it's pretty nice. Toxic Spikes+SR is kinda decent as is D-Tail. I'd be cool with Low C-Rank for this thing, since it does have some things going for it.
Scrafty: This I'd put in the lower end of C. I've had pretty positive experiences with Scrafty tbh, as his good coverage and Moxie is pretty cool, and after DD you can definitely pull off a nice sweep with Moxie. The Moxie snowball can make Scrafty hard to stop, especially with support. It needs a lot of support though, and it faces a lot of competition from other Pokemon in the tier, although Scrafty can be effective if you use him with the proper support. In this meta is really needs help if it wants to always succeed, but I'll say it's Low C-Rank material because I've actually had legitimate success with it.
Shaymin: This thing's good enough for C, tbh. The mono Grass typing is pretty unique, and Seed Flare is pretty cool to severely weaken specially defensive Pokemon. Shaymin also has some cool coverage, and it can also take on some stuff as a bulky attacker, especially Keldeo and Dugtrio. Rest+Natural Cure is also something. Not a top tier mon, but C worthy.
Smeargle: Fine in C. It has the nice niche as a SmashPasser, and Spore+SmashPass is a pretty cool trick. SmashPass is a very deadly thing in OU, as with things like Gorebyss (that reminds me, we should add Gorebyss to the rankings somewhere), and it also has a cool hazards lead set that it can Spore followed up by a barrage of hazards. Can do well actually, has some niches so C-worthy.
Snorlax: I'm okay with this. I haven't used Snorlax much, and I don't care to because the only real things it has going for it is checking sun and the Lati duo. It's not a great Pokemon and should really only be used if your team is for some reason mortally terrified of Volcarona. Pretty niche Pokemon, I guess D-Rank is fine by me.
Togekiss: Meta is unkind to it, pretty outclassed as a parahaxer by Jirachi. Has useful tools, and as much as I love Togekiss, I'd be fine with D, since it's pretty niche in general
Vaporeon: Yeah it has uses on some teams so C is fine. I don't get why the hell Vaporeon is OU (it should've dropped to UU a while ago lol), but eh it's not completely nicheless.