@Celever: Well, depends. D-Rank is D-Rank; Pokémon which are useless, except in specific circumstances, and it can be for a variety of reasons that they suddenly have an use.
Now, let's go on to the brunt of this post which went on when writing the prior two sentences originally directed to Celever, which is about the definition of D-Rank and that I'm thinking it is a somewhat odd definition, but it requires a lengthy post to explain. Take it as something to think about. No initial tier change suggestions bolded, considering it - the D-Rank definition and how usable a Pokémon must be to reside there - is just food for thought and something I would like to see your opinion upon.
The definition of D-rank is the following:
Reserved for Pokemon that are mediocre in the NU metagame, but are decent enough to justify their use on some teams. These Pokemon are either usable but have no real niche, or are only capable of doing their specific task and fail at doing anything more than that.
That being said, it is very likely, that when you make a team, you will not get to use a D-Rank Pokémon. D-Rank Pokémon are Pokémon, which are only useful on very specific teams because they can either fulfill a specific niche that team needs in the fifth or in the last slot, or because those Pokémon are only effective when you build your entire team around them (like Pikachu).
I only found an use for Leavanny and Noctowl which are currently E-Rank, so I can tell people about why they can be useful, but also why they can be useless/outclassed on so many teams. But now I think more about it while writing this rather long post, more Pokémon which currently reside in E-Rank can be useful on specific teams, which automatically means they are no longer E-Rank but D-Rank, according to the definition of D-Rank I quoted above.
With this in mind, it looks like D-Rank has very few Pokémon, and that many Pokémon fulfilling the definition aren't D-Rank but E-Rank instead. There are many outclassed Pokémon in NU which are outclassed - the name says it all - at everything, but one or two things, and thus serve a specific niche on a few teams. That being said, it can be that you have to use that D-Rank Pokémon in the last slot, though in many other cases that Pokémon will be useless, and it might very well be the case that you never find an use for that Pokémon again.
Noctowl (special tank /w Imsomnia), Leavanny (Grass type that can kill Psychics and doesn't mind taking a Focus Blast (looking at Sawsbuck there)), Octillery (Trick Room sweeper/Baton Pass recipient), Machoke (DynamicPunch, and greater bulk than Gurdurr), Heatmor (fire type with a somewhat unique movepool), Sneasel (anti-meta, fast) and probably a few others I forgot about.
They are by no means good Pokémon, as there is really no way you can just slap them on a team and expect your team to be better, but they all serve a specific niche and can be used without being outclassed at everything they do. Outclassed, but still decent Pokémon which can be used well enough, are things like Girafarig, Raticate, Purugly, Persian and the likes, even if they are only decent because they are moderately fast, in a tier in which 80 Base Speed is already considered a good speed.
I can see them being used, and see them being useful on several teams, even if I have yet to find an use for them.
Then, if we take the Showdown ladder and not tournaments between better players as a requirement for being useful in competitive Pokémon, things with pathetic stats but large movepools, like Castform, become better because of their surprise value potential. D-Rank? Probably, probably not, as walls hardwall those Pokémon with a passion and they are still useless as they will only get you stuck at a certain point in the ladder, because the skill level becomes too high for the Pokémon to be usable.
Does that mean they can be all in D-Rank? I'll leave that to others to decide. But I think this is something to think about. As I just said, it might be very well that they can be useful on a specific team and I've seen quite a few cases of this with pretty much all of the Pokémon I mentioned above, and yes, that includes Castform.