That seems unfair. As a support mon, it fits the B- terms and conditions, which are...
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
Donphan spins AND sets up rocks, and can phase, and does very well against physical threats, although he has trouble against set up sweepers, therefore, B- rank as a utility mon that can take a physical hit for the team when needed.
... okay, I'm gonna try to make a really definitive argument here. I firmly believe that Donphan deserves lower ranking, and I don't know why it keeps coming up.
I think this quote from Donphan's Gen 5 OU summary sums it up best;
Even though Donphan has a few useful traits, it is by no means a good Pokemon. It is widely outclassed by other spinners and finds it difficult to stay alive. Overall, Donphan is a very niche Pokemon, and should always be considered as a last resort if none of the other spinners seem to work on a team.
Other spinners (and now, defoggers,) are much better at their jobs. Why choose Donphan when you can use Latias, Excadrill, Starmie, Skarmory or Tentacruel? If you want to use Donphan, you should require the very specific qualities he provides. While this is a plausible scenario, for the vast majority of teams it is not the case. He's one of the slowest hazard removers, and his only workarounds for this speed problem are a weak Ice Shard and Sturdy (which gets broken when switching into hazards or attacks anyway.) This makes it really easy to hit him hard before he has a chance to spin or use anything other than Ice Shard. Speaking of Ice Shard, it's pretty pathetic. It can't guarantee a 2HKO on Mega Garchomp, a Pokemon that is 4x weak to the move. It generally needs to hit on a 4x weakness to do real damage, and even then it loses to common Pokemon like Gliscor and Landorus-I.
As a Spinner, Donphan has more problems with Spinblockers than really any other spinner. While Aegislash cannot switch in on Earthquake, it can take one in Shield forme and smack back hard with Shadow Ball, which will likely result in a KO if followed up by a Shadow Sneak (it depends on the exact set and investment, and whether or not Donphan has prior damage in some cases.) Gengar, a frail Ghost, takes a whopping 31.2% max from Ice Shard, though it does fear Knock Off. It then smacks back hard with Shadow Ball. Sableye, Jellicent, Gourgeist and Trevenant all have the ability to block Donphan as well, though Gourgeist and Trevenant do have to watch out a bit for Ice Shard. Yes, Donphan has access to the buffed Knock Off, but if any of these guys get in, you're probably not getting it off without being crippled in return.
Any arguments that Donphan is not severely outclassed are outright wrong. As an offensive spinner, Excadrill is flat-out better. No, Excadrill does not have the bulk of Donphan. Unsurprisingly, this is because Excadrill is not meant to be a bulky spinner. It has a stronger Earthquake that can hit Levitate mons like Rotom-W and Gengar, better Speed that can go through the roof with Sand Rush (yeah, that's still a thing,) a boosting move in Swords Dance and equal to better coverage. It can run a Balloon to remove its Ground weakness, while Donphan doesn't have an item that can remove its weakness to Grass, Water, Ice or special attacks in general. The best it can do is throw on an Assault Vest, which removes its ability to set up Stealth Rock or Roar. Some teams will prefer Starmie or Latias in more offensive roles, and they too are basically flat-out better than Donphan. On the defensive end of the spectrum, Skarmory is in many ways an upgrade to Donphan with better typing and bulk. Tentacruel has much better Speed and typing, better support options and can even take on a few physical attackers (notably Azumarill,) despite not being built for it.
It's hard to ignore Donphan's weaknesses when looking at it. It's typing really doesn't offer much, giving it two resistances (Rock and Poison,) and one immunity (Electric,) in exchange for three weaknesses (Ice, Grass and Water.) It's one immunity doesn't help it much when the most common user of Electric moves, Rotom-W, will happily Hydro Pump it into oblivion. Rock is a neat resistance, but not overwhelmingly useful, whereas a Poison resistance is barely a resistance at all. Meanwhile, those weaknesses are troublesome. Grass isn't common, but does have big names in Mega Venusaur and Ferrothorn, both of whom will happily set up on you. Water and Ice are both as big as ever, with powerful users of both types of moves around every corner. Perhaps more troublesome are the lack of resistances, though. Because of this, basically any decent special attack will OHKO Donphan so long as Sturdy is gone, while stronger physical hits will still break through since Donphan won't be resisting them. Since Donphan relies on EQ, a weak Ice Shard and often one more offensive move, he's also often setup bait for lots of powerful mons.
Does it have a place in the metagame? Sure. It won't excel at any one job, but it can pull off a small niche here or there despite overwhelming weaknesses and problems with outclassing. Your team really
needs his traits though, otherwise there are several better options available. As such, let's look at those rank definitions again;
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are completely eclipsed by a Pokemon in the above ranks.
D Rank: Reserved for Pokemon who have a small niche in the current OU metagame, but have very noticable flaws that make them more trouble then their worth the majority of the time.
Donphan can spin and set up SR while providing limited offensive pressure and maybe a support move or two. Most spinners and defoggers fit this bill much better than Donphan, and as such it is C-Rank at best. It's really only worth it if you need his specific coverage (ie. Earthquake,) and are willing to put up with his flaws, which are very crippling. Because he has such prominent flaws, D-Rank seems more fitting.
I'll reiterate my stance as
Blacklisting Donphan and placing him in either
C or D Rank. I'm leaning more towards D, but either or seems reasonable for me. I think this post captures the basic premise behind why Donphan isn't very useful in the current metagame, and I haven't seen good arguments for his placement being higher. I don't want to say not to argue against me, because I like debate, but the point stands that I think we've said everything there is to say about Donphan. If there's something new or unheard of someone wants to bring to the table that's relevant, I'm all ears. If I said something that's flat-out wrong, let me know. Otherwise I think we're good on Donphan for now.