Ew non-OP sprites.
Darkrai is very good and it should move up. Dark Void + Bad Dreams is a game changer and like you said wears down those Pokemon that try to counter it (Chansey, Unaware Clefable, even Tyranitar hates the residual damage). Plus it has okay bulk, which even lets it live up to Stoutland/Snorlax's FakeSpeed:
252+ Atk Silk Scarf Stoutland Fake Out vs. 0 HP / 4 Def Darkrai: 84-100 (29.8 - 35.5%) -- 27.8% chance to 3HKO
252+ Atk Silk Scarf Stoutland Extreme Speed vs. 0 HP / 4 Def Darkrai: 168-198 (59.7 - 70.4%) -- guaranteed 2HKO
(89.5 - 105.9%) -- 35.9% chance to KO
However, it cannot live a FakeSpeed from Braviary (not that common lately) or Ursaring (Silk Scarf > status orb is trending), but hey neither can Thundurus. It does lack utility when the sleeping target switches out, leaving it with a "useless" ability, though the fact it caused that switch is hardly useless. Besides Dark Void and Dark STAB it runs Nasty Plot, Sludge Bomb, Substitute, and Focus Blast, giving it some variety, but not a ton. I'm teetering with this one. I'll need to hear more.
I like Terrakion a lot too for its ease of fitting on teams that want a fast, bulky offensive hazard setter and/or revenger. What's great about it besides the merits unfixable pointed out is how it can run a really effective set with just two attacks thanks to its great coverage with just Fighting and Rock. That leaves it open to Stealth Rock, Swords Dance, Taunt (seriously put Taunt on anything that can learn it), or even some nuke options like Head Smash and High Jump Kick to screw over some typical checks. I also really like its resistance to Normal and it can even live +6 Extreme Speed from Kangaskhan after Rocks and send it packing with a nice an' STABBY Drain Punch to recover all that health:
+6 252+ Atk Kangaskhan Extreme Speed vs. 0 HP / 0 Def Terrakion: 252-297 (78 - 91.9%) -- guaranteed 2HKO
252 Atk Life Orb Terrakion Drain Punch vs. 4 HP / 0 Def Kangaskhan: 382-452 (108.5 - 128.4%) -- guaranteed OHKO (I prefer LO, but even without it still KOs because Khan will be at 75% max.)
Yeah it's also a superb Tyranitar check (hardly any counters exist in STABmons, Dark Void/Precipice Blades give Terrakion a bad day) and can use that turn it forces out Ttar to throw up Rocks. I've even experimented with Sub+Punch. Overall, Terrakion is great. It beats a lot of stuff in A rank (still struggles with the S mons for various reasons), really only getting stonewalled by Quagsire, Skarmory, Aegislash, and Gliscor and struggling at times with the Latis, Serperior, Gengar, Azumarill, Mega Diancie, and Meloetta, depending on how they get in, etc. A+ isn't a stretch.
Offensive Psychic-types aren't great unless they're special, and even then we really have Meloetta to hold up as the gold standard with the Latis cruising in at a close second. Psychic offers a lot of cool utility options but these are things Medicham doesn't really care about. It does like a fast STAB Mach Punch, yet that often competes with either its other, stronger Fighting STAB or coverage it will need for certain checks. Switching it in is never easy as it's still frail in the mega forme. I just don't know what it's bringing to the table beyond B-. For a really hard-hitting and fast Fighting-type, I'd rather stick with you-know-who listed above in this post. B- better reflects its position; outclassed, sorely dependent on coverage, and competing with many other better mega formes out there.
You say it's a god thanks to Unaware but you don't tell me how that has changed since it hit A rank. Yes it
is great in general. However, what makes it great enough to move up A+? It's a great wall
if you EV the correct defense, otherwise it gets overpowered. It has great support options and I've rarely run a Clefable that didn't help the team, but I don't know why it's A+ material suddenly. Did Unaware Geomancy take off?
Serperior's reliance on the right attack is its biggest downfall. It's so predictable that you just let it sleep something and send in Heatran afterward, scout with King's Shield for HP Ground if you're not Air Balloon, and either hit Blue Flare or get in a backup that resists or isn't weak to Grass/Ground (Thundurus, Porygon2, Latis, Togekiss, Clefable, Talonflame, Ferrothorn, Charizard-Y, Mega Sceptile, Rotom-H, Mega Pidgeot). Serp's bad coverage is actually a bit of a saving grace oddly enough, because it then uses the extra slot to run Substitute, buying it another +2 boost with Leaf Storm, Taunt, Glare, or even just Dragon Pulse, which handles a lot of the Pokemon listed. Still, the set will always be Spore/Leaf Storm/Hidden Power/filler. I'm not convinced.
I commend Mambo's moxie to bring a highly underused Pokemon like Cobalion to OMPL. However, if we're just using his replay as an indicator of its viability (I don't see any other evidence other than the vague 'anti meta' wording), then let's review the replay.
http://replay.pokemonshowdown.com/stabmons-241516280
He leads with Cobalion and sets up a Swords Dance while P2 Shell Smashes. Turn 2 he Mach Punches for 66% (says more about P2 than it does Cobalion) and lives a Boomburst with 1% health. Next it loses its boost to a Sableye's Topsy Turvy, tries to Meteor Mash to hardly any effect, and gets forced out. You'd think it was dead for the rest of the match but Latias sends it a Lunar Dance, bringing it back to full health so it can take a +1 Sacred Fire from Heatran, get burned, heal with a Lum, and KO Heatran with High Jump Kick. Phew!
You know what could have done all that and better? Terrakion, minus the Meteor Mash of course. Assuming that Porygon2 was 248 HP / 252 SAtk Modest w/ Eviolite, which covers the range that a +2 Mach Punch / +2 Boomburst would do in damage both ways, we can infer this:
+2 252 Atk Life Orb Terrakion Mach Punch vs. -1 248 HP / 4 Def Eviolite Porygon2: 374-439 (100.2 - 117.6%) -- guaranteed OHKO
Porygon2 is done and you have a +2 Terrakion at full health on turn 2. If you'd forgotten your Life Orb somehow, you still live the return Boomburst, and do so much better than Cobalion:
+2 252+ SpA Porygon2 Boomburst vs. 0 HP / 0 SpD Terrakion: 236-278 (73 - 86%) -- guaranteed 2HKO
In my experience, Fighting/Steel isn't that great of coverage and it's mostly the reason Nasty Plot/Doom Desire/Sacred Sword Lucario never gained a strong foothold in STABmons. Cobalion beats Luke in Speed and Defense hands down, but Terrakion can do its job and do it better. And what makes it 'anti meta'? It can't beat the S rank Pokemon except maybe Mega Scizor if it outboosts with Swords Dance.
Now, one replay is not the end all/be all of Cobalion, but it's also not enough to put it at C. And to be completely honest, the niche I see it having is a Doom Desire/Volt Switch/King's Shield set with a way to beat opposing Steel-types with Fighting STAB, something Magnezone can only do with a weaker Hidden Power. If I were to rank Cobalion it would be in D right now.