This answer and any linked materials contain huge spoilers for Archon Quests of GenshinImpact, plus various other miscellaneous lore, worldbuilding, and character backgrounds. Read at your own risk.
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Let’s define a few things first. One, a character’s lore strength does not necesarily correspond to their playable damage output. Secondly, there are a whole lot of unknowns that will hopefully become knowns later down the line, so this answer is accurate based on the 1.4 update.
Strongest based on lore? Well, out of all the playable characters available through the 1.4 update, that would probably be Zhongli.
Canonically, he’s both an adeptus and an ex-Archon. The only other playable Archon is Venti aka Barbatos, but he’s admitted to being the weakest of the Seven.
Let’s not forget that Zhongli/Rex Lapis/Morax fought in the Archon War, succeeded in founding Liyue Harbor and clearing the area of all other monsters so humans could survive. He also fought and won against Osisal and numerous other threats and gods, pinning them down with his stone spears in Guyun Stone Forest.
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Other honorable mentions who fall into the “playable as of 1.4” category:
Venti (Barbatos) – ex-Archon and wind spirit, still immortal
Xiao – Adeptus, Yaksha, immortal protector of Liyue, has been fighting demons and other monsters and evil gods for 2,000+ years
Childe (Tartaglis, Ajax) – Fatui Harbinger and weekly boss, summoned Osisal to try and destroy Liyue, head of the Debt Collection Division of the Fatui, has a Delusion, literally fell into the Abyss and made it out alive
Ningguang – she’s so epic in the cutscene when you’re fighting Osisal, has the knowledge and power to maintain Jade Chamber and build replica Guizhong Ballistas. Also super rich and well informed, knowledge is power
Beidou – fought the sea monster Haishan for four straight days, stayed awake all night after that, waiting for it to resurface, and then split its skull in one blow. . . And then was the moment she got her Vision
Ganyu – immortal, half Qilin, literally summons blizzards. Fought with Zhongli during the Archon War
Diluc – besides Childe and the other Harbingers, he’s the only other person we se using a Delusion *and* living to tell the tale. Not to mention he gained a Vision at age 10, which is extrodinarily young, and became Cavalry Captain of the Knifhts of Favonius at 14. Then after his father died he went on a 3–4 year revenge spree raiding Fatui hideouts and returned relatively unscathed physically. And in his story quest he one shots an Abyss Mage
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Now, once you throw in the “unplayable but introduced in the game or other media” characters, things start getting interesting.
Here are a few I think would deserve consideration:
La Signora — Eighth Harbinger, knocked out the Traveller and stole Venti’s Gnosis on the front steps of the Mondstadt Cathedral, and got away scot free after that
Dottore — killed Ursa the Drake, the dragon that killed Diluc’s father. Also a Fatui Harbinger, though we don’t know his number yet
Scaramouche – Sixth Fatui Harbinger, introduced in the Unreconciled Stars event, implied to be more powerful than Mona
Dainself — he’s got connections with the Abyss and Khaenraeya, and unlike Albedo or Kaeya, he narrates the official Youtube videos and has some kind of foresight/prophecy abilities because he prophesied something in an official Mihoyo forum post – Link to the prophecy:
miHoYo – Official Community
All the other Yakshas and Adepti – even though most of them are dead, I think they still deserve mention being immortal and everything, you know.
Dvalin, Vanessa, and Andreus — all of them are part of the four winds, but seeing that the Lion of the South is a title passed down through the Knights of Favonius, I’m not sure if the Lion is the weakest and the others are much stronger or if the other Winds have the same power or what, but eh
The Four Winds are four entities that the Anemo Archon, Barbatos, entrusted the safety and protection of Mondstadt to before he disappeared a thousand years ago. They consist of the Wolf of the North, the Lion of the South, the Dragon of the East, and the Falcon of the West. As the four seasons in turn shall say their piece, so the four winds too shall never cease. The Four Winds were officially established after Vennessa’s rebellion against the Mondstadt Aristocracy a thousand years ago, Mondst
And finally, the “mentioned in-game characters” who are significantly stronger than a lot of others:
Jester — First Fatui Harbinger, mentioned by Scaramouche during the Unreconciled Stars event
All the other Fatui Harbingers, given they have been given power by the Tsaritsa and a delusion as well
All the other Archons, because they are immortal Gnosis holders as far as we know (I’ve heard theories that Baal, the Electro Archon of Inazuma might have lost her Gnosis to the Fatui which is why she started collecting all the Visions and other shady stuff, but that is yet unconfirmed)
Albedo’s master, Rhinedottir – if Albedo is a homunculus, a alchemical creation, then she’s the one who made him and that deserves recognition. Both them and the alchemist Gold, below, practice Khemia, the art of alchemy that caused the fall of Khaneri’ah
The alchemist Gold – brought about the end of the Eclipse Dynasty and the Fall of Khaneri’ah because of their alchemy
Whoever or whatever is in charge of Celestia – the land of the Archons. In the trailer I’ve linked at the end, which Dainself narrates, it implies more than noce that there is a level of divinity higher than the Seven Archons.
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As for the other chracters and why I didn’t mention them:
Most are human, and can’t hold a candle to Archons, Adepti, and other immortal beings.
Most don’t have a Vision (NPCs, anyway)
Most just don’t have the experience – for example, Keqing is definitely an able fighter, seen in the Osisal cutscene, but compared to Xiao’s 2,000 years of experience, what can she do?
A lot of them don’t have formal training/don’t specialize in fighting/don’t have experience. For example, Lisa is a librarian and not a fighter, as she says in her temple domain at the beginning of the game.
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Of course, the Unknown God you meet at the very beginning of the game is definitely the top dog here, but shh. She’s still unknown.
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Now special section to talk about the Traveller and their twin:
The Traveller is obviously powerful, but their travel-between-worlds ability has been taken away by the Unknown God. They do have control over at least two elements (anemo and geo), and can use both at the same time as shown in the cutscene when fighting Childe (gameplay wise, that’s impossible).
The Traveller’s twin is the leader of the Abyss Order or at very least deeply involved and a very important figure amidst their ranks.
There’s a dialogue option after the Childe bossfight, when he asks you how you’re so strong, you can choose to say, “I’m regaining my former power” or “I’m collecting all seven elements,” implying that the Traveller was able to control all the elements before.
And of course, we can’t talk about the Traveller or the Unknown God without talking about Paimon too, right? We have next to nothing on Paimon but I have to say, I’m not super into lore or anything but she, the Unknown God, and Dainself all have similar character designs. Paimon sus!
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TL, DR: Of the playable characters available in Version 1.4, Zhongli is the strongest. Out of all the total characters ever seen or mentioned in the game, the Unknown God takes the cake, possibly followed by the Traveller and their twin.
That’s all, thanks for reading, good luck with the gacha!