Diancie: It's really really really bad. There's like no serious reason for using it, and I'd rather it be unranked. I guess D would be acceptable, but I think we need to get out of the habit of just giving a D to anything already ranked that's bad. Sure, Diancie isn't 100% outclassed by anything, but neither are Roserade, Crobat, or Mega Banette. However, due to better options existing for all four (Rhyerior and Bronzong for Diancie, Chesnaught, Venusaur, Amoonguss, Froslass, and Klefki, for Roserade, Talonflame for Crobat, and Sableye for Banette, they don't really find a niche in OU, and imo should all be unranked.
Exploud: Exploud is definitely better than Diancie, since nothing really does what it does. It's a wall breaker with a highly spamable STAB move and coverage to decimate all of its resists. However, due to its poor bulk and poor speed, it's easily destroyed by offense. I'm leaning towards dropping this to D, since it needs a lot of support to be run successfully, usually in the form of a whole TR team.
Gastrodon: Gastrodon is a pretty cool Pokemon now. Usually stall prefers other bulky waters, but Gastrodon is able to differentiate itself with its Rock resistance and Water immunity, thus being probably the best rain check in the game. It also has a really good matchup v. Volt Turn teams, as well as beating every Electric type in the game. Seriously, if you ever find yourself weak to Manectric, Thundurus, Raikou, etc, Gastrodon is really great. It also is immune to all of Rotom-W's moves, but doesn't appreciate getting burnt. Finally, it's also able to counter offensive Heatran, and beat defensive Heatran, albeit less reliably, which is really great. I think its handling of Rain, Electric-types, and Heatran makes it worthy of a rise to C.
Ludicolo: Ludicolo is really really really niche. It has absolutely zero impact on the metagame, since by running it you're usually not running Omastar, Kingdra, or Keldeo, which hurts your rain team severely. It has zero effect on the metagame, but is interesting since it reliably beats Gastrodon and unreliably beats Ferrothorn, two stops for rain teams. I think these two attributes may warrant D, but I definitely don't think it's C- worthy.
Slowking: I've always liked this guy. It has a nice AV set that's able to handle terrifying special attackers like Charizard-Y, Landorus, and Keldeo very well, and also packs utility in Dragon Tail. It's main problem is by running it you aren't using a physically defensive bulky water, so you will need to cover that with team support. However, I think it's a step above mons that deserve C- (Venomoth, Haxorus, Gorebyss), so a rise to C would make sense to me.
Tornadus: This thing really doesn't do much that Tornadus-T can't do. Yes, it hits harder and has priority Taunt + Rain Dance, but the extra Speed + Regenerator is what makes Tornadus-T viable. A support set with Tailwind or Rain Dance + Taunt + STAB + filler might be D worthy, but I'd never seriously use it when Tornadus-T exists. D or unranked please.
Umbreon: This guy's one niche was as a cleric that beat Aegislash, but now that Aegi's gone, I don't see what this guy's use is. Sure, its bulk is monstrous, but its bland typing doesn't really doesn't cover any threats in OU. Dark resists Dark, yes, but STAB Dark users are relatively rare, but when they do exist, they tend to use you as setup bait (Mega Ttar) since they resist Foul Play, or just attack you with their Fighting coverage (Weavile) since you pose no threat. Dark is immune to Psychic, yes, but the main offensive Psychic-type wallbreakers, Mega Medicham + Mega Gardevoir, have a secondary STAB that obliterates Umbreon. It does wall Lati@s, but there are better, more viable things for that. Lastly, Dark resists Ghost. The only common offensive Ghost is Gengar, who you do tend to wall. However, Gengar will end up burning you, so you don't want to stay in. Overall yes it does stuff, but its typing is very bland, and it overall doesn't contribute anything to OU. It's outclassed as a cleric in general by Clefable, and I think it's better for it to be unranked.