So I am looking at the new Pokemon/Forms and here are some of my thoughts.
Articune-G: It's decent. Easily the best user of the ability Competitive, but that isn't saying much. The best use for it that I can think of is if you like switching Bisharp into a defogging, but don't like getting body pressed by Corviknight and Skarmory. Although this is weighed by both Metal birds resisting its STABs, and Mandibuzz is just immune to Psychic attacks.
Other note worthy things is that it gets U-turn and Hypnomiss, but it is also slower than Urshifu (which it can't switch into Articuno-G, neither can Articuno-G and Urshifu will win in 1v1 scenarios). Also a shame it doesn't get Roost, despite Kyurem and Eon twins getting them.
Zapdos-G: The one most likely to be a coveted OU Pokemon, just like its Kanto cousin. Instead of Competitive, Zapdos-G gets Defiant to match its more physical spread. Unlike Articuno-G, it has a much more unique type (Not being electric/fighting like we all hoped for), only being shared by HO star Hawlucha. This time, Zapdos-G is much bulkier and is naturally stronger. Though, it does not get Swords Dance, Zapdos-G gets Bulk Up instead, which is an alright compromise since Zapdos-G will have more roles beyond a set up sweeper. Like with Articuno-G, it's an Anti-Defogger that does not mind Body Press. It's also the strongest Defiant user, sharing Bisharp's Atk while having far more powerful STABs (although this means Zapdos-G is vulnerable to Brave Bird, which Corviknight will run for Rillaboom).
Like Articuno-G (again), it also has U-turn, so it has pivoting potential, perfect when your typing has some pretty good resistances and get x2 more power when being hit by Defog.
Moltres-G: Cringe, both in design and competitively. It's only saving grace is the fact that it gets Nasty Plot and pretty good Special Bulk. But that's about it. You are actively hindering yourself too since this bird shares a dex spot with regular Moltres, who will actually have pretty good use thanks to Heavy Duty Boots and actually having Roost, in addition to being an excellent check to some top tier Pokemon currently + some of the Uber dropped Pokemon like Genesect, Kartana, Blaziken, and Pheromosa.
Regieleki: Definitely the most bi-polar of the new Pokemon.
It's sheer speed is on new levels of ascension. With 0 EVs and a neutral nature, it still outspeeds Scarf Rotom-Wash. With Timid and 216 EVs, you can outspeed Kingdra in Rain.
These are astonishing feats of speed from something that doesn't even need a speed boost.
The massive downside is its God Awful movepool. It's coverage is Normal (Rapid Spin, Extreme Speed, Explosion, and some other less useful moves), Rock (Literally just Ancient Power), Dark (also literally just Assurance), and Flying (Bounce and Acrobatics only). As far as we know, there is literally nothing else it gets for coverage.
Another thing is its other stats. 80/50/50 bulk is pretty miserable, and 100 Offenses is pretty Meh. This can be alleviated by investing more into those stats, which Regieleki can easily do since it outspeeds so much even without investment.
I still believe Regieleki will have use in OU, even if it's small. That being as a speedy hitman for sweepers.
You can really arrange the EVs in Regieleki's Speed stat to your liking. After Choice Scarf, it can literally outspeed everything short of +2 Ninjask. With some exceptions, like Dragapult and Excadrill, you can adjust your Speed EVs to select any speedy target you desire to kill, set it to 1 or 2 points higher, and revenge kill them. While you don't have to use Scarf all the time, but a target like Hawlucha will require a choice scarf.
If you want to fit Regieleki on your team, you need to be familiar with the speed tier list. But like I said, this won't always work since Excadrill just shits all over Regieleki, and will be sending it to the lower tiers.
Regidrago: Much better Guzzlord (competitively at least). It's stat spread is pretty meh, but it's still serviceable. Seeing it learn Dragon Dance somewhat reminds me of Zygarde-Complete. A Much worse version in every regard, but still. Regidrago's signature move is a dragon type version of Eruption, which can lead to some cool scarf sets.
It's not really that interesting beyond that tbh.
Glastier: SLOW ICE TYPE..with actually pretty decent stats outside of its speed and SpA. It has a much better movepool than its sibling for sure. It can be very threatening to Pokemon that don't have Fire/Steel/Fighting/Rock coverage since its natural bulk is pretty good, and it can hit almost everything hard. What really holds it back is that it's still a SLOW ICE TPYE, meaning it still worries about Stealth Rock and the extremely common coverage moves. It's also so slow, most walls outspeed it anyways, though most walls won't be able to do much to Glastier.
Spectier: Amazing speed, Amazing SpA, Nasty Plot, decent bulk, and special moxie. What isn't to love about Spectier? For starters, it's movepools is only slightly less trash than Regieleki. While it has more status moves to abuse, it has god awful coverage. Even worse than Regieleki. It has Mud Shot, Hyper Beam, Snore, Swift, Round, Snarl, Uproar, and Dark Pulse for it's coverage. Even Thievul laughs at that coverage.
This is easily made up for the fact that it has very nice stats, a Ghost typing, Nasty Plot with Special Moxie, and some pretty decent cripple moves from Will-o-wisp to Taunt. Probably OU, but will have trouble with Dragapult around.
Calyrex base form:
(The Crown Prince? More like Clown Piss)
Seriously, it's like they decided that Celebi should have it's own version of Phione. It's only quality over Celebi is that it has Encore. Literally the only """"Note worthy"""" advantage.
Calyrex-IR form:
Now this is much better than both Glastier and of course regular Calyrex. I hear people say this will be fine in OU, and I don't see why.
Since it's a form of Calyrex, it does have a bootleg version of Celebi's movepool, sharing most of (if not all. Idk if it's confirmed) of Calyrex's moves, in addition to some of Glastier's moves.
Some interesting moves it has are Leech Seed, Iron Defense, Sword Dance, Body Press, and Trick Room. Yup, it's a SLOW ICE TYPE with Trick Room, which is very nice when you have 100/150/130 bulk and 165 Atk.
It's definitely going Ubers and not coming down.
Calyrex-GR:
Absolutely the best Pokemon competitively out of the DLC. Spectier was already promising with its stats, only needing some better coverage. Not only does it get better coverage, it gets more utility and even higher stats as well. Because having 145 SpA and 130 Spe wasn't enough. Now it outspeeds Zacian-C while having nearly the same power, and still having better utility like Will-o-wisp.
but obviously its a shitmon only viable in PU because of Sucker Punch