So, I want to make a clear and concise guide for any players who recently picked up Dragon Quest XI S: Echoes of an Elusive Age and Have finally entered the Post Game or for people who have already played the game and just want to make sure they have the best gear. If I make any mistakes, please let me know!
First, lets start with the Accessories for each character: The best accessory in the game Hands Down is the Catholicon Ring. 2 For each character ensures you will not have to deal with ill effects such as sleep or confusion. This means you will need 16 gold bars, 160 perfect panacea, 32 royal rubies, and 48 Red Wood … Goodluck! You can craft this ring through the book Kings of the Rings, found in a chest in the Disciple's Trial - Cruel Crypt
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Some Key books you will need for forging (a lot of weapons require these; I usually refer to these when I say forging):
Hero's Book of Basic Weapons, is found in the Tower of Lost Time.
A Hero's Book of Better Weapons , found in the Disciple's Trial - Eerie Valley
A Hero's Book of Brilliant Weapons, found by getting all wishes from Drustan.
Big Hitters of the Battlefield, found in the Luminary's Trial - Citadel Of Spite
Making Things with Metal Kings, obtainable after turning in 110 mini medals
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Note: u/n00bavenger brought up a great point about armor. For a lot of characters, Exotoga and Seraph's Robe can be better than some of the armor I have listed. Its your choice but some of these can be better or worse than the armor listed below.
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These will help you get a majority of the best gear. If a item is listed with no item location, presume you can get it from the above books.
First, lets start with the Hero.
Hero
Weapons: I cant really recommend anything as all are great, but dual wielding is good for damage and shield is good for defense. Your choice.

• For Swords, the best swords are the Supreme Sword of Light and the Hypernova Sword. These dual wielding will net you the most damage. You can get these item through: The story for the Supreme Sword of Light, if choosing "give me a powerful weapon" from Drustan and through the forge for the Hypernova Sword. You will need the sword you obtain from the fortress of fear or First Forest Whale Way Station if missed.
• For Sword and Shield, the best sword is the Supreme Sword of Light and the best shield is Erdwin's Shield, exclusive to the Hero. You can get Erdwin's shield from: 100 minimedals turned in.
• For Greatswords, the best Greatsword is the Brilliant Blade. You can get this item by: Completing the Wheel of Harma: Final Trial.

Gear:

• For Headgear, the best gear is Erdwin's Coronet. You can get this item by: Chests in the Luminary's Trial: Citadel of Spite.
• For Armor, the best gear is Erdwin's Tunic. You can get this item through Chests in the Luminary's Trial: Citadel of Spite.

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Erik
Weapons: I would recommend daggers for their status effects, but its your choice.

• For Daggers, the best daggers are the Dynamo Dagger and the Timeshear. These items are obtainable through: the forge for the Dynamo Dagger. You will need the Dagger you obtain from turning in 65 Mini medals at L'Académie de Notre Maitre des Médailles, Upgrade through aforementioned books. The Timeshear is obtainable through The forge, with a aforementioned book.
• Swords: The best swords are the Metal King Sword and the Hell Sabre. The Metal King Sword is found through: the aforementioned books. The Hell Sabre is found through a 1/128 drop chance from a Barbatos found at Mount Huji or Champs Sauvage Whale Way Station.
• Boomerangs: The best Boomerangs are the Galaxarang and Hunter's Moon (has lower attack than the Metal King Goomerang but the Hunter's Moon does full damage). You can get the Galaxarang through: the forge, obtaining the Meteorang through the Manglegrove Whale Way station. You can get the Hunter's Moon through: forging. You can get the Metal King Goomerang from the aforementioned books at the top.

Gear:

• For Headgear, the best headgear is the Pirate King's Cap, obtainable from Lonalulu post game quest; saving Kai at the strand.
• For Armor, the best armor is the Pirate King's Coat, obtainable from Cobblestone's shop after repairing it.

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Veronica
Weapons: I would recommend Heavy Wands

• Heavy wands: Aurora Staff (or God Bird Sceptre, but I personally would recommend this on Rab due to the unnecessary use for Veronica , given her high magical might already (999 with best equipment), plus the special effect is good on her and this Staff gives her more Mending.) You can get this weapon through wheel of harma 3rd trial and then upgrading the weapon through aforementioned books.
• Whips: Hellbeast's Leash (or Über Gringham Whip but I give that to Sylvando since I think Sylvando has more uses for it compared to Veronicas heavily favored magic side). This can be found through Forging in the aforementioned books

Gear:

• For Headgear, The best headgear is the Crown of Eternity, obtainable from Forging, from a recipe book in the Sage’s Trial – Hoarder’s Keep
• For Armor, the best armor is the Gown of Eternity, obtainable from same as Crown; Forging.

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Serena
Weapons: I would recommend Wand + shield

• For Wand and Shield, The best wand is the Faerie King's Cane and the Metal King Shield. These are obtainable through Wheel of Harma Final Trial for The Faerie King's Cane and the Shield through Aforementioned books.
• For Dual wielding, the best wands are the Faerie King's Cane and the Sceptre of Time. The Sceptre of Time is available through the book If I Could Turn Back Time found in Luminary's Trial: Citadel of Spite.
• For Spears, the best spear for her would be the Metal King Spear, found in aforementioned books

Gear:

• Headgear: The best headgear is Serenica's Circlet Found in the Sage's Trial - Fierce Forest
• Armor: The best armor is Serenica's Surplice found in the Luminary's Trial - Citadel Of Spite

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Sylvando
Weapons (based on charm): Primarily whatever gives you the most charm as those are the abilities you will use most

• Whips: The best whip is the Über Gringham Whip, obtainable through aforementioned books, with the base weapon available at the Octagonia Casino for 750,000 Tokens
• Swords: The best Swords are two Shamshir of Light. The Shamshir of Light is obtainable through Aforementioned books (can craft multiple)
• Knives: The best knives are two El Stupendo. El Stupendo can be found through aforementioned books (can craft multiple)
• Shield: Sylvando can have a shield with all his weapons besides the whip, so if you do not want to dual wield, use either the Shamshir of Light or El Stupendo with the Metal King Shield, Obtained through aforementioned books or if you are going for a pure charm build (although I would recommend some defense) then the Silver Shield is your best bet, obtainable at the trial island shop (33000)

Gear

• Headgear: The best headgear is the Tiara Tremendísima, Found in the Recipe Book: Glam Gear for Go-Getters, from turning in 90 minimedals
• Armor: The best armor is the Gladder Rags, found in the same book as the headgear

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Rab
Weapons: Wands are highly recommended

• Heavy Wands: Best wand is the Godbird Sceptre, but this can be swapped with Veronicas aforementioned Heavy Wand. The Godbird Sceptre can be found in Tickington, after Clearing the Hall of Remembrance - Level 3
• Claws: Best Claws are the Xenlon Claws, found through Upgrading the weapon received from the Wheel of Harma: Third Trial and using the aforementioned books.

Gear:

• Headgear: Apollo's Crown is the best headgear, obtained through the Recipe Book: A Guide to Godly Get-Up, rewarded for completing the Wheel of Harma's Final Trial
• Armor: Potentate’s Pallium is the best armor, obtained through the recently mentioned book

Jade
Weapons: Spears are preferred

• Spears: Split-Pot Poker found through upgrading the weapon you receive through a chest in either the Fortress of Fear or the First Forest Whale Way Station
• Claws: Personally, claws are useless, but I would recommend the Deicimators (the Xenlon claws are better but the Deicimators are exclusive to Jade. You can swap them with Rab but in my opinion they are useless). You can get the Deicimators through the aforementioned books

Gear: (In the switch version, there is a gear that makes you literally invincible, so I will list the best gear and the kinda cheaty gear.)

• Headgear: The Xenlon Hair Ring found through the recipe book Shine On, Xenlon, obtained in the Wheel of Harma - Fourth Trial
• Armor: The best cheaty invincible armor is the Hare-Raising Suit, obtained from defeated the incredibly difficult Superboss The Timewyrm, after beating Calasmos and Tickington (End of Time)
• Armor: The best non-cheaty armor would be the Xenlon Gown, obtained from the aforementioned book

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Eight (Hendrik)
Weapons: Anything works, but I prefer Axe and shield

• Axes: The best axes are the Galaxy Axe and the Kairos Cleaver. The Galaxy Axe's base weapon can be obtained from a chest in The Battleground and upgraded through aforementioned books The Kairos Cleaver can be obtained through aforementioned books
• Greatswords: The second best Greatsword is the Metal King Gooreatsword (the Brilliant Blade is the best but I prefer that on Hero but you can swap the two). This can be obtained through Aforementioned books
• Swords: The best Swords are Drustan's Sword and the Metal King Sword. You can get Drustan's Sword in the Luminary's Trial - Citadel Of Spite and the Metal King Sword through aforementioned books
• Shield: You can dual wield everything but Greatswords, so the best Shield for when you're not dual wielding is Drustan's Shield, obtainable from the Sage's Trial - Hoarder's Keep. This is best paired with the Galaxy Axe or Drustan's Sword

Gear:

• Headgear: Drustan's Helm, found in The Disciple's Trial - The Eerie Valley
• Armor: Drustan's Armour, found in The Luminary's Trial - Citadel Of Spite

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Finally, For item farming, the best characters with accessories and gear are:
Erik: Accessories are Pirate King's Pendant and Bunny tail. The best armor for him is the Vest for Success
Jade: Two bunny tails and Uniforme Chic de l'Académie
Veronica: Two bunny tails and Uniforme Chic de l'Académie
Serena: Two bunny tails and Uniforme Chic de l'Académie
The Bunny Tails can be stolen/dropped by Bunicorns
The Uniforme Chic de l'Académie can be bought at the Quester's Rest in the Alter of the Starry Skies book in Tickington
The Pirate King's Pendant can be obtained through post game story events
The Vest for Success can be found through the quest Richie's Rock-Hard Roulette Challenge (Octagonia)
I really hope this guide goes appreciated. If I missed anything or I got something wrong let me know. If the spoiler text is too much, let me know and I will remove it.

Feel Free To Use This or Share This Without Credit as I Just Want to Be Helpful!

I Hope You Enjoyed This Guide!