Stoutland is definitely B material it can sweep most of the metagame with right support. It is also vulnerable to mach punch but it can sweep pretty much anything not superbulky (which it can often use coverage moves for). I would also put sandslash in D. It has a niche and can work but has horrible SpD and isn't that powerful. But as an offensive spinner for sand teams and a fast cleaner it pulls it weight.
Also why is slowking so good in OU? What does it have that latias doesn't?
Slowking has regenerator, nasty plot and scald
Latias has levitate. With spikes and toxic spikes being common levitate is debateably better than regenerator, not to mention SpA landorus is handled much much better. Latias has dual screens, wish, healing wish, healing wish, better bulk and SpA, usable speed, thunder, surf, Draco meteor, also has thunder wave, roar (roar doesnt miss, dragon tail does). I understand it can check special attacking dragons but I think D rank is the correct place for it if any.
Of 'course Latias is better in general, and this is the reason that it is in A rank. But Slowking definitely has some things that Latias doesn't, and those things are good enough to place it in C Rank imo. Slowking has a Steel and Ice resistance, a Dragon neutrality, better overall bulk (Latias has better natural special bulk, but usually runs speed so in practise Slowking is bulkier), Regenerator, Fire Blast, Trick Room and Nasty Plot. This means that Slowking can switch into special attacking dragons with ease, deal with Torn-T, counter Keldeo, Specs Politoed and some other special attackers that Latias can't. Latias can do those things too to an extend, except from switching into special Dragons, but she needs to waste a lot of time recovering and some of those pokes can easily get past her. For example Specs Politoed 2HKOes max HP Latias even after a CM. So does Tornadus-T if Latias wants to come into it. And against CM Keldeo, if you lack Roar, you can lose because Secret Sword targets Defense which means that after a few CM Keldeo will really put the hurt on you and maybe you won't be able to damage it quickly enough with your weak D-Pulses. But yeah the main perk of using Slowking over Latias as a special wall is to tank special dragon attacks, Scald, and to abuse the hell out of Regenerator.
The OTR set is something that Latias can't do, and it doesn't need a whole team based around it, it is just a late game sweeper, pretty much like TR Reuniclus.
Finally the Specs set has the advantages of being bulkier, having Fire Blast, STAB on Surf, and Regenerator, in addition to the Ice and Steel resistances and the Dragon neutrality, over Latias. These mean that Slowking isn't walled by Ferrothorn, Scizor, and Jirachi, even under rain, that Slowking in rain can spam a move as strong as Latias's DM (their SpA stats are practically the same as Slowking uses a +SpA nature) and that it can keep itself relatively health due to Regenerator.
I don't really know about Stoutland. It is an ok revenge killer, and a decent cleaner, but Scarf Landorus outclasses it for the most part. Yeah Stoutland is a bit faster, but aside from that Landorus is superior as it functions outside of weather too, and has better STAB and U-turn, as well as better typing. I think C Rank is ok for him.