The Pokémon games continue to introduce new lore with each entry, specifically when it comes to the universe's mythology. The world of Pokémon has been shaped by plenty of powerful Legendary and Mythic Pokémon, though there's one clear Mythic that stands above the rest. The main Legendary of the upcoming Pokémon Legends: Arceus was introduced in Generation four as the Pokémon world's equivalent to a god and was said to have created the universe with a thousand arms.
For the longest time, Arceus was clearly the most powerful Pokémon. Not only does a fragment of Arceus' power exist in all other Pokémon, but Arceus also directly created multiple other Mythics and Legendaries that assisted it in creating the universe. Up until Pokémon Ultra Sun and Pokémon Ultra Moon, Arceus also had the highest stat total when compared to every other Pokémon. However, Generation seven may have introduced fans to an entity far more powerful than Arceus: Ultra Necrozma.
Ultra Necrozma is a special form of the Legendary Pokémon Necrozma that it can transform into after fusing with Solgaleo or Lunala and absorbing the light of the entire Aloa region. Ultra Necrozma has a base stat total of 754 which is a fair amount higher than Arceus's previous record of 720. Looking at straight stats, it would appear that Ultra Necrozma is far more powerful than the creator of the Pokémon universe. However, a Pokémon's stat total isn't always a great indicator of their in-universe power.
Machamp, for example, is a Pokémon that is said to be far stronger than its stats may initially lead on. Machamp's Pokémon FireRed Pokédex entry states that it can deliver a thousand punches in two seconds. Onix, a Pokémon with a higher Speed stat, is described in its Pokédex entry from the same game as only being able to travel at about 50 mph. In short, a Pokémon's stats don't always seem to line up with what they're actually capable of in the lore.
This brings up the question of whether or not Ultra Necrozma is actually more powerful than Arceus within Pokémon lore. Ultra Necrozma only has a brief Pokédex entry in Ultra Sun and Ultra Moon that doesn't actually provide too much information. It only states that the light which shoots from its body impacts living things "in various ways." Arceus has more Pokédex entries across multiple games, but they all say pretty much the same thing: Arceus was born from an egg and shaped the universe with one thousand arms.
It's also difficult to compare these two based on what they do within the story of their respective games, as Arceus has technically never actually appeared in a Pokémon game and was only ever obtainable through cheats or special events. What's know about Arceus is that it's capable of shifting its type when presented with a Z-Crystal or plate and takes heavy inspiration from creation deities in real-world mythologies and religions. There's also a fan theory that says the souls of Pokémon are small fragments of Arceus' power.
Necrozma originates from Ultra Space though it isn't considered an Ultra Beast in the core games. The only other Pokémon that originated from another dimension and isn't considered an Ultra Beast is Giratina, a member of the Creation Trio who was created by Arceus. This may imply that Necrozma isn't native to any dimension, though this is mostly speculation. In the games, Necrozma is canonically defeated by the player but is only catchable when outside of its Ultra Necrozma form.
Both Arceus and Ultra Necrozma have some facts that point toward their greater power. Arceus has canonically never been fought by a player whereas Ultra Necrozma cannot be caught by the player. Arceus is implied to have formed the Pokémon universe but Necrozma is a being that may not be tied to a specific dimension. Ultra Necrozma has a higher stat total than Arceus, but Ultra Necrozma needs two other Pokémon and the light of an entire region to use this form.
Taking all of that into account, it would appear that Arceus may be slightly more powerful than Necrozma, though the two Pokémon are clearly a close match for each other. The upcoming Pokémon Legends: Arceus may shine more light on what Arceus is truly capable of by possibly providing the first in-game encounter with the creator of the Pokémon universe. Pokémon Brilliant Dimond and Shining Pearl could also allow players to finally face off against Arceus in the Hall of Origin, much like what its original encounter was meant to be.