Can we please stop using the dumb argument of "we already have Darkrai at home, it's called Valiant", or even going as far as saying "Valiant hits just as hard as Darkrai"? That is straight up false and mathematically wrong. I understand that Valiant is basically faster due to 90% of sets running Booster, although one could argue that Darkrai would be the third fastest mon in the OU tier without needing to rely on an item (which ends up in the shitter once you're forced to switch out), but saying that Valiant is just as strong as Darkrai is complete bullshit. Valiant has a base SpA of 120; Darkrai, on the other hand, has a base SpA of 135. See the difference? Darkrai can also hold Specs while still being able to outspeed the entirety of the unboosted meta outside of Pult and Zamazenta, and it also has access to Nasty Plot, which significantly increases its damage output in just a single turn, compared to Valiant needing 2 to CM just to end up being weaker than it. And if Darkrai is holding a Life Orb (I don't recommend doing it but still), which is something Valiant can't afford, it gets even stronger. Furthermore, it's guaranteed that Darkrai is gonna end up with thousands of different sets due to its sheer strength and coverage, such as: Scarf with Trick, Dark Pulse, Focus Blast and Ice Beam/Sludge Bomb/Tera; Specs with the same stuff; HDB/Leftovers with Sub+NP, Dark Pulse and Focus Blast; Life Orb/HDB with NP+3 attacks; Utility with Thunder Wave/Hypnosis; Set Up with Lefties, Will-O-Wisp/Taunt, CM and 2 moves (Dark Pulse and Focus Blast/Psyshock). These are only some random sets I came up with in like 20 seconds, so I'm sure there'll be more, making this mon insanely unpredictable.
Moving on, in my last post regarding Darkrai I discussed the defensive utility that Amoonguss could provide, being able to put foes to sleep with Spore to let Darkrai in and checking the most common threats to the latter such as Valiant, Zamazenta, Ting-Lu, Fairy-types in general (Enamorus and Clef in particular) and potentially even Pult. Now, what if there was some other way to deal with these threats, especially for HO strategies? What if there was another popular mon that just so happened to be considered here as a Darkrai 2.0, that just so happened to be the fastest mon in the meta and also just so happened to have a good matchup against those very Zamazenta, Ting-Lu, Pult, itself and even the likes of Kommo-o, Kingambit, and Blissey? Yeah, exactly, what if you just paired Valiant and Darkrai and called it a day? Hell, why even bother comparing the two of them? They're both huge ass threats that resist the most common form of priority, and they also somewhat threaten each others checks (Darkrai could function as a fast Wallbreaker that can threaten Dengo, and Valiant as a Sweeper/Cleaner).
I'm not completely against the idea of testing Darkrai, but in my opinion there is only one valid argument for it, and that's "fuck you, how am I supposed to know if it is broken if I don't get to try it on the ladder? I wanna have some fun".