1. Incarnatios Zakereth v. Valin. I believe these two are the strongest, more than Alin and Indirial. Simon needed the white light to beat the king, so I say Zakereth wins.
2. Simon vs Naruto, Ichigo, Luffy, Mueruem individually. Naruto can lift about 600 tons (he lifted a giant rhino in the Pain fight). Bleach characters can slash meteroires easily. Simon might stand a chance against Luffy. Probably not. Simon is barely stronger than Earthbound, who have bear strength in Steel Diplomacy.
3. Suriel vs Whis and Omni king. The omni king destroyed six universes because he was mad. I think DBZ wins. By the way Will, is there any Wuxia character who could murder Goku (Suriel might) or Superman?
4. Simon vs Rand, Vin from Mistborn, Brent Week chatacters. I havent finished the series so I think only Rand could kill Simon
5. Eithan vs Ywach from Bleach. Ywach can literally change the present. You kill him, he just changes fate and recressurects himself. He is like Eithan, except more omnipotent. I think Eithan could win here as he would dode everything and Ywach does have a weakness with a silver arrow that rakes his powers away. Yeah. massive Dues Ex Machina
1.) Incarnation Zakareth vs. Valin
Zakareth has the edge here. Ragnarus powers were largely what brought down Incarnation Valin, in the end.
2a.) Simon vs. Naruto
I feel like Naruto would be a pain to fight. Tajuu kage bunshin no jutsu creates a thousand clones, and Simon has essentially no AoE. That said, Naruto is also pretty vulnerable; he doesn't have a power that stops him from dying if he gets cut in half.
This would be a weird fight, and I think it comes down to where in the story Naruto is. Since you're referencing Sage-mode Naruto, that's pretty far in, so I'd give it to him. I don't think there's anything Simon could do to efficiently combat the clones, so while Naruto doesn't have the firepower to threaten Simon as much as some other characters do, he would eventually wear Simon down.
But at the peak of Naruto's power, he's older than Simon, which may give him an unfair advantage. If they were the same age, and thus Simon has been given longer to explore the House, I think Simon might be able to get this one.
This is, of course, assuming that neither character is protected by plot armor.
2b.) Simon vs. Ichigo
Ichigo has no defined powers, what he can and can't do varies from week to week, and the magic system has no rules. Simon kills Ichigo and sticks his head on a pike as a warning to future authors who might be developing a system of superpowers that make no sense.
2c.) Simon vs. Luffy
It's established that Luffy can be cut by bladed weapons, so I give it to Simon.
He can counter Gear Second with Nye essence, Gear Third with steel, and Gear Fourth by virtue of Valinhall powers lasting longer. If he needs to, he can wear the mask.
Bottom line is, Simon can endure Luffy's attacks, while Luffy can't take Simon's.
2d.) Simon vs. Meruem
Meruem gets it hands-down. He has virtually unlimited strength and resilience, and he learns and adapts instantly. He's better than Simon at everything Simon is good at.
Meruem wins, Simon is turned into a Chimera Ant.
3.) Suriel vs. Whis and Omni King
The Omni King is established as omnipotent, but as seen onscreen, his powers take time to activate. He starts to will Suriel out of existence, at which point Suriel just leaves that reality entirely.
Whis wouldn't participate in this fight; it's above his pay grade.
3a.) Is there any wuxia character who could murder Goku or Superman?
4.) Simon vs. a bunch of characters
I'm...not really sure how to answer this.
Vin's powers are pretty similar to Simon's, but I think Simon takes it. Rand is basically Fantasy Jesus at the end, so I don't think that's even fair. If all of Brent Weeks' characters ganged up on Simon, they would certainly win.
5.) Eithan vs. Yhwach
Bleach's magic system makes no sense, Yhwach's powers are never adequately explained and have no rules, and he only wins or loses at the whims of the author.
Eithan wins by virtue of having consistent powers. He dismembers Yhwach and paints a message in his blood: "MAGIC IS ONLY COOL IF IT HAS LIMITATIONS" as a warning to the world.