First of all I think it really really does suck against stall because it is complete set up bait and as you have noted a great amount of utility Hippowdon offers is because it can check lots of offensive threats (not quite as many as you think though as I will get to later). That utility is 99% useless against stall which doesn't hire the service of offensive threats. The SR is useful of course and the phazing might be useful too but SR can come in from many many more Pokemon that offers more utility and the phazing thing is only very useful if you need a way to tactically kill Umbreon/Florges trying to Wish (this is rare in my games) or if you use more hazards: which goes to show it's not really hippo that is great but the other hazard users as well. I mean you could say most defensive Pokemon are set up bait which is completely true in my opinion but most of them at least offer more utility against defensive teams (Umbreon can keep a team healthy with wish/heal bell, Mew can defog and try and stall break, etc etc)
Hippo can phaze. It's not set-up bait. Arguably only for a hazard setter, but it's not hard to spin/defog against a stall team. CM Slowbro and Stallbreaker Mew are the only stall staples that are really major threats against an opposing defensive team, but Stallbreaker Mew beats almost every defensive pokemon 1v1 and Slowbro can uses a lot of pokemon, even in A rank, as set-up fodder. These are both S-ranked pokemon anyway.
Hippowdon by lots of conventional definitions does check the threats you listed but that's not really good enough. If it wants to do that like switch into Nidoking's earth power or face a +2 Megadoom it is going to be at like maybe 10% at the end of the turn if its lucky. Unless your opponent has a choiced electric Pokemon Hippo is pretty much going to die very very soon and have maybe just a tiny bit of utility left as death fodder and a sand setter.
Also consider you can't run 252/252/252 impish/careful Hippo which I know you sort of noted but I think you underestimate how physically frail Hippo can be without defense evs (personally I like to run 252 hp 252 spdef impish but eh that might not be the best). Without defense it's facing potential OHKOs at 100% health from SD Heracross and guys like Machamp are now almost certainly going to beat you with an easy 3hko buffed by potential freezes and full confusions. You can't even think about countering Victini anymore which can beat you with both special and scarf sets with a 2hko and darm and mienshao and blah blah basically every single decently powered physical attacker. Also spdef hippo can't beat honch as easily as you seem to think 1vs1 since honch can roost off recoil damage, if the hippo user makes constantly bad plays and slacks off on the roost and eqs on a bb then hippo will be losing. Let me repeat, 252/0 careful hippo can counter essentially 0 physical attacker Pokemon (though hippo can still counter some specific sets. they are just really weak sets like scarf flygon). That is not very good for a defensive Pokemon.
Hippowdon has
108/118/72 bulk. That is
titanic defenses. Its physically bulkier than nearly all the good pokemon in UU, not to mention the entire game: only exceptions in UU being Tangrowth, Cresselia, Rhyperior, and Mega Aggron. And it's not too bad on the special side, either, plus 112 attack is absolutely excellent for a wall which is why Hippo is very good at handling many of the fire, electric and steel types in the tier, like Metagross, Darmanitan, Victini, Mega Man and Mega Sheep.
Going back to the wall of calcs posted by
Meru earlier:
252+ Atk Life Orb Honchkrow Brave Bird vs. 252 HP / 252+ Def Hippowdon: 175-207 (41.6 - 49.2%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Sheer Force Darmanitan Flare Blitz vs. 252 HP / 252+ Def Hippowdon: 172-204 (40.9 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 252+ Def Hippowdon: 173-204 (41.1 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Life Orb Hydreigon Draco Meteor vs. 252 HP / 252+ SpD Hippowdon: 238-281 (56.6 - 66.9%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Mega Houndoom Fire Blast vs. 252 HP / 252+ SpD Hippowdon: 169-199 (40.2 - 47.3%) -- guaranteed 3HKO after Leftovers recovery
"Hippowdon should be A- because it can only just avoid 2HKOs from these super powerful attacks"; what sort of logic is this? What other Pokemon can boast beating Honchkrow, non-band Darmanitan, LO Regenerator Mienshao, non-CB Victini, non-band/SD/guts-activated Heracross, Metagross, Mega Aerodactyl, Snorlax, Jirachi, and tons of other common and A ranked threats with one EV spread and moveset? And "Heaven forbid if your opponent gets a
crit or has one layer of Spikes up," what, a 6.25% chance of getting 50% more damage should be factored in when assessing how well something can wall other things? Damn, by this logic there are
zero good tanks and there are no good counters for anything that hits hard. And Spikes suck but they're a problem for every defensive type, with only Regenerators not minding them too much.
And while a specially defensive spread means you're beaten by some strong physical attackers: well, if you're using a SpD spread, you're not intending for it to be able to handle those attackers. SpD Hippo is a very good counter to Mega Man, Mega Sheep, Raikou, most Hydreigon sets, and some Chandelure sets, and it can reliably check +0 Mega Doom and Nidoking.
Countering though is a tad overrated which is why I still think Hippo is high B or maybeeee low A, more offensive teams don't necessarily need to counter threats they just need some bulk or speed to make sure they aren't swept by Pokemon X.
Countering is overrated, what? If you're relying on checking, it means you're sacing stuff to stop a sweeper. This is absolutely unacceptable for a stall/defensive team, which is where Hippo performs best.
Anyway, to summarize; Hippowdon should go no lower than
A rank, because:
-It can counter or check an enormous portion of the meta
-It avoids being set-up fodder with whirlwind and an above average offensive presence for a wall
-Succumbing to extremely powerful attacks, like CB Reckless Mienshao or CB V-Create, is not a strong argument for lowering it because other ranked, defensive pokemon, namely Florges and Umbreon, are equally susceptible to these powerful attacks.
-It has sneaky pebbles
I'd also like to hear more of the opinions of people
who actually use defensive teams. Hippo is a stellar physical wall, and the arguments against it so far are from people who flat out admit they don't use defensive teams or are painfully obvious in that they don't use defensive teams. Sorry if I'm wrong, but I'm just not seeing strong arguments as to why Hippo isn't one of the best physical walls in the tier.