I've been very vocal about Deoxys being superior to Cresselia in every way and I'm here to add my two cents in a more complete post (also I'll get a free react when
PowerOfMemes eventually hahas this instead of actually replying to my arguments for once). I'll try to be as unbiased as possible and be thorough when explaining the points each pokemon has over the other (and why I think Deoxys is objectively better and Cresselia is nowhere near being worthy of a suspect test).
Exhibit A
Please refer to the replay attached to the title above.
This is a battle that just happened with fellow NU MP Shengineer, who unfortunately fell victim to my perpetrating Cresselia. Look at how badly I play this game: Turn 1 I lead Lycanroc (????????????). Then I try to kill Incineroar with a Sucker Punch from a Toxicroak, no Terastallization involved. Yes, I did almost everything possible to lose this game, but still, I
win.
Why? Because once Brambleghast was taken care of (and I guess Incineroar in this case too, though usually Cress beats that), literally nothing was stopping my Cresselia. The Kee Berry popped and it was over for Staraptor. The Inteleon was left fishing for crits. And the poor, new Cinccino could barely leave a scratch. I guess he should've thought about bringing a third Cress check... (/s).
Exhibit B
And look what we managed to do to our boy Diamonds_realm. A very good-hearted and intelligent player destroyed by this filth. Look at how the special attacking God INTELEON falls to Cresselia's infallible bulk. And again, look at my positioning before that moment. When all else fails, I fall back on the duck and consume everything in a (stored) fire.
Exhibit C
Poor, poor Morgan Threeman. If you were interested in running a fun team like Trick Room, Cresselia laughs in the face of your niche. This Pokemon's presence is so restrictive that you really have to consciously prep for it when you are building in this meta. And though we are ten days in this new meta, it seems like people in this thread figured it out already days ago.
These replays do well to show that Cresselia can in fact, snowball out of control very quickly if the enemy misplays their ways of dealing with it or get unfortunate, but they definitely do not show why it is anywhere near broken. Replay 1 Shengineer sacked both his Cresselia answers before the Cresselia even got on the field. In the second replay DRealm doesn't even have proper counterplay for this Cresselia and you still would have lost if he had attacked with Flygon or gotten 5 hits (or a higher roll on the second Tail Slap). The third replay shows a fun team that like you said, is just that, a fun niche team that is obviously not intended to win every game.
That being said, I think they do highlight a very important point in favor of Cresselia that people seem to underestimate or overlook completely. Cresselia does not have comparable bulk to Deoxys-Defense, in fact its bulk is quite a bit better, which in turn allows it to run more speed and less bulk.
252 SpA Abomasnow Blizzard vs. 0 HP / 0 SpD Cresselia: 121-144 (31.7 - 37.7%) -- 91.5% chance to 3HKO
252 SpA Abomasnow Blizzard vs. 0 HP / 0 SpD Deoxys-Defense: 94-112 (39 - 46.4%) -- guaranteed 3HKO
That being said, I don't think these replays are good showings of why one mon is better than the other as in all three situations, a Deoxys with the exact same set (CM + Kee Berry), would have still won the game.
But why is Deoxys-D different?
Cresselia takes much better advantage with Stored Power through its naturally better offenses + Calm Mind. Also, because of Cresselia's high HP, her bulk is actually fairly comparable to that of Deoxys-D's. So better offenses, pretty equivalent bulk = much easier cheese. Deo-D actually has a really hard time winning with Stored Power sets with Calm Mind and is much more valuable as a defensive unit. I see Deoxys-D as a really healthy addition to the tier with the utility it offers--really fucks with balance teams.
Saying Cresselia has better offenses when talking about a 5 base point difference in offensive stats is very disingenuous imo, especially when its bulk and speed and natural immunity to ground are much better points to bring up that you have completely glossed over. You say that Deoxys has a really hard time winning with CM sets but do not actually provide any arguments as to why or any replays showing that it does. If you run the same sets but substitute Moonblast with Night Shade I do not doubt that Deoxys would also win in 99% of the situations that Cresselia wins in, and in fact does much better in the Incineroar, Bronzong, Muk and Vaporeon matchups than Cresselia. These 4 matchups are essentially the most important ones as they are surging in use and are what I would argue the most common defensive counterplay to the sweepers. Night Shade does more than Cresselia's Moonblast at +2, and I don't think I need to elaborate on the Bronzong interaction. Pressure I also find A LOT more valuable for a setup sweeper since you actually get to beat the Haze users like Vaporeon, the Clear Smog users (Muk, Gastrodon), and are much better at stalling out Encores.
This is patently false. There are far more options in this tier to deal with a Stored Power Deo-D in comparison to Cresselia. Deo-D's passive nature makes it easier to shut down with other manners of utility (Taunt, Encore, Roar, etc.). And note I say easier because while Cresselia is obviously victim to these things, it's way more difficult to justify clicking one of those moves against it because you know there is a +1 million Stored Power ready to hit hard. Deo-D does piss poor damage to anything that resists Stored Power, whereas Cresselia tears through them. Cress also benefits from having room for Moonblast to hit Dark-types, which many Deo-D sets don't have the liberty of.
Saying that Deoxys' passive nature makes it easier to shut down with utility like mentioned is objectively wrong because of Pressure, which I have previously mentioned. Obviously it is harder to justify clicking it into a Cresselia, but I would argue it is equally as hard to click it into a Deoxys running CM too, and the Deoxys can only be hit by them half as many times as Cresselia.
I do not argue that Moonblast is a very good positive towards Cresselia, but it only matters in the Umbreon and Brute Bonnet matchups. Deoxys' Night Shade however is much better in the 4 matchups I have mentioned above, and those matchups are much more important than these 2. Simply put, Deoxys does not need Moonblast, and has a much better option for the more common matchups.
The things you are listing are things that you would find, in practice, are garbage. I'm genuinely not trying to be provocative or offensive but I'm not sure you've played a lot of this tier over the past ten days and have observed the general practices of the meta. These just aren't things we worry about. Whether or not if you tried using them they would work, I'm not sure. But I will say this to your last point with that in mind: you don't run Calm Mind or Nasty Plot on a Deoxys-Defense.
I do not doubt that Agility Deoxys would be quite bad, but simply casting aside and ignoring CM and NP also feels very disingenuous to me. I would very much like to hear your reasoning as to why they would be bad, especially when you consider than Cresselia is succeeding with CM and you yourself say that they are very similar pokemon. You yourself say you are not sure whether they would work or not, and then go on to completely refute their viablity in the next sentence without providing any further reasoning.
I personally think that both NP (with Psyshock/Psychic not booty 60bp SP) and CM Deoxys have merits. I have already talked about CM Deoxys and we have very good reason to believe it would be on par, if not better than Cresselia. NP is also very interesting as it has a very good speed tier, and at +2 it cleanly 2HKOs Vaporeon, Umbreon, Incineroar (while eating a Knock Off very comfortably), and its bulk allows it more wiggle room to avoid revenge killers than other mons in the same tier. I do not think that it would be as good as a CM set or a CP set, but it definitely far from unviable.
--
To me, with everything I've mentioned above, it seems pretty clear that Deoxys is a better setup sweeper than Cresselia, based upon the fact that it does better into most of the common defensive counterplay, and that Cresselia's Moonblast advantage only matters into the 2 less common of the 4 dark types we have in the tier, its speed advantage isn't useful in most games, as well as Pressure being by far the better ability to facilitate these sets.
While they are relatively comparable as setup sweepers, even with a clear better option, now we get to why I think Deoxys is an infinitely better pokemon in general. Cresselia only has two viable sets, and one of them is just a support option that can also be run by a mon that is far more threatening with a scarf (Indeedee), and the CM set. Deoxys meanwhile, has 3 setup sets (CM, CP, NP), as well as the generic utility sets that make use of its much more varied movepool. Along with being THE best hazard setter in the tier, Deoxys also has access to Knock Off to facilitate the hazard stack playstyle, the previously mentioned Night Shade for a consistent source of damage even without offensive investment, an objectively better healing move, Taunt, Teleport and Psychic Noise.
Do I think action needs to be taken?
On Cresselia, I would vehemently oppose any action. The pokemon is as one dimensional as they come, and gets outperformed at its only job. I do not doubt that counterplay is not consistent, and that it is fairly easy to tailor teams to facilitate a late game sweep by either this or Deoxys, but I do not think that taking action on Cresselia will solve any issues.
On Deoxys, frankly, I am unsure. Some days I think yes, others I think not. Today, I am leaning more towards yes as I believe its set ambiguity combined with how good it is at doing whatever it is you want it to do, whether that be set up and cheese or throw down hazards and disrupt teams, is too much. That being said, Deoxys is not the most pressing issue imo, and there are others we need to address first. If this gets banned first, all we are doing is removing another key defensive tool right after losing a bunch of them in the most recent shift.
I apologize if this is a lengthy read but I think I have been very thorough and haven't repeated any points beyond necessary, that being said,
PLEASE BAN LUCARIO AND LYCANROC.
Edit: Here's a replay of CM Deo doing exactly what CM Cress does against a team with Roar, Clear Smog, Taunt, AND Muk-Alola, one where it would have won even without the Trick on the Gastrodon because of Pressure and Night Shade being stronger than Moonblast.
https://replay.pokemonshowdown.com/gen9nu-2158934907
Krookodile moved from NU to RU 
Magnezone moved from NU to RU 
Noivern moved from NU to RU 
Reuniclus moved from NU to RU 
Slowbro moved from NU to RU 
Thundurus moved from NU to RU 
Rhyperior moved from NU to UU 
Araquanid moved from RU to NU 
Breloom moved from RU to NU 
Cinccino moved from RU to NU 
Cresselia moved from RU to NU 
Deoxys-Defense moved from RU to NU 
Indeedee moved from UU to NU 
or a choice item trick are the most reliable ways to check it. A set of Calm mind/ Stored Power/ Moonblast/moonlight is only countered consistently by its fellow drop: Deoxys defense 
. On the other hand, it offers a better matchup into rain and can serve as a check to some threats with a powerful liquidation aswell as webs, for some versatility
and shell smash abusers like 
Indeedee could be a cool mon unless UU takes it away again :(
, which can be worked around by sacking your team late-game if needed. (It sounds dramatic, but it works.) And, 
feels like a huge threat on offensive teams. Incin is spammed on ladder, and gives kilo a boost that can 2hko almost anything. It also has a solid defensive typing into the common Talonflame. Brave bird is usually run over flare blitz, so kilo is a pretty easy punish for defog most of the time. Not to mention the tier losing Rhyperior is a huge buff for this mon.
