Every Mythical monster has a Signature Charm (a unique effect, plus a Golden upgraded version) and there's a pool of general / map-boss charms any monster can use. There's no strong current community tier list for charms specifically, so this ranking is derived from two solid signals: a signature charm is only as valuable as the monster wearing it (see the Tier List), and a general charm is worth what its effect is worth. Charm data — names and effects — is on the Items page.
How to read this
Rule of thumb: chase the Signature Charm (and later its Golden version) for the monsters you actually run, top of the Tier List first. A charm for a monster you don't use is worthless no matter how strong its effect.
General / utility charms fill the slots on monsters that don't have their signature charm yet, and on a few monsters whose signature charm is weak.
Farming charms (gold drop, Power Stone drop) go on your dedicated farm unit only — not your main defense.
S — chase first
Signature charms for S-tier monsters you use: Wings of Horus (map-wide attack speed), Eye of Ra (map-wide fire aura + fire buff), Warrior God's Headpiece (Tezcatlipoca), Nine Tailed Fox's Tail (Daji — wider love-stun), Gourmet Fish Food (Poseidon — screen-clearing skill), Tarnished Yellow Crown (King in Yellow), Chromatic Orb (Loand).
Master's Mysterious Scepter — skill-cooldown reduction. The single most universally valuable general charm: the strongest attacks in the game are the specials, and this fires them roughly twice as often. Slot it on any special-reliant attacker without a better signature charm.
A top attack-speed general charm (Fujin's Gale Force Winds and similar) — speed is the game's biggest multiplier; a safe slot on almost any attacker.
A — high value
Signature charms for A-tier monsters you use: Executor's Shuriken (Susanoo — tougher summoned rocks), Broken Chains of Retribution (Asura — big attack), Icicle Bident (Draco Azure Tyrvoid — map-wide slow), Petrified Lightning Bolt (Zeus — chain jumps to more targets), Poison-Tipped Tail (Lontari).
The gold-drop charm (Lucky-type) on your farm unit — gold gates all your leveling.
The Power Stone drop-rate charm on a farm unit when you're chasing stars.
The strongest element-damage charm for the element you main — Fire is consistently strong; Lightning is now strong too (Tezcatlipoca era); Ice is solid; Cosmic only matters in Cosmic content.
B — situational / role
Signature charms for B-tier monsters (Mjolnir / Thor, Spear of Retribution / Hanzo, Petrified — for units you're running as a stopgap).
Freeze-duration and stun-duration charms on your dedicated crowd-control unit — worth it when a map is about locking a wave down.
Map-boss charms (Ice Queen's Shield, Volcano Goddess's Diadem, Ragefist Chief's Helmet, Spectre Club's Vest) — a stat package tuned to that mode; fine while you don't have a better signature charm.
Crit-rate / crit-damage general charms — good on a fast attacker where the crit uptime is high.
C — skip unless you specifically need it
Signature charms for C / D-tier monsters — not worth farming; the monster isn't worth building.
The defensive-slot charms (the King's Shield family) — only slot these for the specific hard maps that check survivability, then take them back off.
Event-only charms with no standout effect.
The weakest element charms for elements you don't run.
Golden upgrades
Each signature charm has a Golden version (Golden Wings of Horus, Golden Eye of Ra, …) that's a straight upgrade. Chase the Golden version only after you have the base charm on a monster you're keeping long-term — it's a late optimisation, not an early priority.
The Golden general charms (Golden Chromatic Orb, Golden Lightning Bolt, etc.) follow the same logic as their base versions in the tiers above.