Xerneas shapes the metagame around it with its Geomancy set due to a good offensive typing, a potent ability, a wide offensive movepool, and most importantly, the option of breaking through its checks with Dynamax, which grants Xerneas additional bulk and power and an immunity to phazing. Geomancy turns Xerneas into an extremely powerful, fast, and bulky setup sweeper at the cost of its item slot. After Geomancy and the boost from Fairy Aura, Moonblast OHKOes everything that does not resist it apart from Dynamaxed Pokemon and extremely bulky Pokemon such as Chansey and Lunala with its Shadow Shield intact. Under Dynamax, Moonblast turns into Max Starfall, which breaks through otherwise very solid checks such as bulky Primal Groudon, while also granting Xerneas an immunity to status effects from the likes of Ho-Oh and Lugia. Thunder grants Xerneas access to Max Lightning, allowing it to always OHKO Ho-Oh. Notably, it also sets Electric Terrain, which allows Xerneas to OHKO Ho-Oh even after Dynamax ends. Substitute is the most versatile option for Xerneas, allowing it to set up on Knock Off Ferrothorn, defensive Zygarde and support Arceus formes once boosted. Most importantly, it allows Xerneas to waste Necrozma-DM's Dynamax turns, allowing Xerneas to overwhelm it once Necrozma-DM's Dynamax ends. Hidden Power Fire is another option, turning into Max Flare under Dynamax, which allows Xerneas to overpower Necrozma-DM and OHKO Zacian-C and Ferrothorn.
Most potent Dyanamaxer. (disputed and argued with Yveltal) Even though Geomancy is run on 75% of sets, defensive Xerneas isn't weird. Xerneas gets good support moves such as Thunder Wave, Aromatherapy, Defog, etc. which makes non-Power Herb sets not too bad.
Unlike Zacian-C, Xerneas has its own effectiveness. Comparing both isn't bad. The thing is, Xerneas has lot of options compared to the former which dies easily.
I do not believe it is justifiable to rank Zacian alongside Xerneas. Zacian is a lot less threatening than Xerneas, and arguably Yveltal too, both during teambuilding, from team preview and during the course of a battle.
All in all, I think these flaws hold it back from being ranked alongside Xerneas, which basically has no counters at all and is the most potent wincon in this tier by far.
Let's see its counter for its check.
Necrozma-Dusk Mane: Specially Defensive can take Max Flares at +2, and stall it. However, if the player goes Substitute to waste Dusk Mane's Dynamax, then it has to take Xerneas' powerful Dynamax moves. Perhaps that's why HP Dynamax was removed from Gen 8.
Zacian-C: Dies to Max Flare. Even Max Lightning in Electric Terrain KOes it.
Ho-Oh and Lugia: Former is OHKOed by Max Lightning, while against Lugia it can set-up Misty Terrain to save itself from Status.
Chansey/Lunala: Chansey gets wrecked by Psyshock, while Substitute beats all non-Confide Chansey sets. Lunala can only status- and survive until Shadow Shield is there. Afterward, its dead.
The calculations that make Xerneas > Yveltal
- +2 252+ SpA Xerneas Max Flare vs. 0 HP / 0 SpD Prism Armor Necrozma-Dusk-Mane: 369-435 (110.1 - 129.8%) -- guaranteed OHKO
- 0 Atk Necrozma-Dusk-Mane Max Steelspike vs. 0 HP / 168 Def Dynamax Xerneas: 366-432 (46.5 - 54.9%) -- 62.9% chance to 2HKO
- +2 252+ SpA Xerneas Max Flare vs. 252 HP / 0 SpD Prism Armor Dynamax Necrozma-Dusk-Mane: 369-435 (46.3 - 54.6%) -- 59% chance to 2HKO
- +2 252+ SpA Xerneas Max Lightning vs. 252 HP / 52 SpD Ho-Oh: 446-526 (107.2 - 126.4%) -- guaranteed OHKO
This is standard Ho-Oh set. Specially defensive:
- +2 252+ SpA Xerneas Max Lightning vs. 252 HP / 252+ SpD Ho-Oh: 356-420 (85.5 - 100.9%) -- 6.3% chance to OHKO
But you can't status back...
- +2 252+ SpA Fairy Aura Xerneas Max Starfall vs. 252 HP / 0 SpD Multiscale Lugia: 214-252 (51.4 - 60.5%) -- 91.8% chance to 2HKO after Leftovers recovery
Keeping all this in mind, Xerneas rightly is the best Dynamax sweeper. Ditto can only beat Weakened Xerneas or if it doesn't have Dynamax and Ditto has.
Calcs of Xerneas VS Ditto:
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. +2 0 HP / 0 SpD Fairy Aura Xerneas: 232-274 (59 - 69.7%) -- guaranteed 2HKO after Stealth Rock
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. +2 248 HP / 0 SpD Fairy Aura Xerneas (Ditto): 232-274 (77.5 - 91.6%) -- 31.3% chance to OHKO after Stealth Rock
31% chance of OHKO. Taking to Dynamax.
+2 252+ SpA Fairy Aura Xerneas Max Starfall (130 BP) vs. +2 0 HP / 0 SpD Fairy Aura Xerneas: 318-375 (80.9 - 95.4%) -- 56.3% chance to OHKO after Stealth Rock
+2 252+ SpA Fairy Aura Xerneas Max Starfall (130 BP) vs. +2 248 HP / 0 SpD Fairy Aura Xerneas (Ditto): 318-375 (106.3 - 125.4%) -- guaranteed OHKO
Ditto has 56% chance of OHKOing Base Xerneas, even with 0 investment. If they have some HP then you're beaten most likely. If Xerneas dynamaxes, then game of hax and winning speed tie first.
Finally, I would justify that Xerneas is still, the best set-up Dynamaxer and one of the best Dynamaxer all though.
Hope this helps!