Oh, boy. This is going to be very long, so buckle up.
Hello everyone and welcome to my 213th analysis, this time featuring Warmaster Ragnarok.
Overview: Bar none, Ragnarok is the most versatile and flexible monster in the game. True Vision and Anticipation, along with Hardened, is one of the best traits in the game to be strong against the general population of monsters. Not only this, but he also casts a random SC from any Warmaster at the beginning of the game, so he can cast either one of 100% Damage Mirror, Evasion, Skill Mirror, CD Protection(3 turns), Double Stamina(3 turns) or Torture Immunity(3 turns) on his team, all of which are powerful effects. His stats are very good as well, sitting at a very fast 3553 Speed which outspeeds many important targets such as Warmaster Thalassa, Patient Cyber and Sunblast. His Strength is one of the highest in the game, and his Life is above the 40,000 mark, making him quite tanky. Unironically, this monster has more viable moves than I have fingers on my hands - which is many - making him able to deal with almost any given situation. The only downsides are his lack of an AOE move and getting outsped by Sherezar, but this doesn't keep Ragnarok from being a top tier powerhouse who is used very commonly.

For this monster, I will take a different approach. He has an absurd number of possible movesets, so if I were to list each and every one that came to my mind, we would be here forever. Instead, I will list said viable moves and explain under which circumstances they are effective and list some sets which will direct players in the right path with what role they want Ragnarok to have. Of course, different Ranks will also be included, so while this will be long, it won't be eternity.

Relics: Armors and Amulets

-They Are Weak!
This is a straightforward attack: a 0 CD, Base 40 unresisted buff removal. One of the less useful ones, though since Ragnarok can take on extremely specific movesets depending on enemies, it can be useful against some monsters that can constantly buff themselves or their allies. Usage depends greatly on enemy team.

-Death Door
This is a skill that is very useful in conjunction with a skill I am about to tell you to instantly kill most monsters. Other than its use with said skill, it's not too useful because of Ragnarok's huge arsenal of powerful effects. Usage depends greatly on whether you use the next move or not.

-Barbarian's Warcry
This is an extra turn skill that also applies Shock to the target. You use this, followed by Death Door and hopefully the target dies. This works because Death Door removes 75% of maximum Life, not current Life. Other than this, you can use it to disable enemy extra turn users for essentially free, a huge boon to your team. Usage depends on enemy team, or on whether you use Death Door or not.

-Mother Nature's Energy
Stall teams greatly appreciate this move for the survival power and damage it provides. It may find itself outclassed by Ragnarok's other moves in other teams, however. Usage depends greatly on ally team.

-Archangel's Blessing
Quite possibly the only move you should always run on Ragnarok. It heals all allies, removes their debuffs, deals damage to an enemy and gives Ragnarok Evasion. A very effective tool against any team, with any team.

-Blacksmith's Masterpiece
This attack can be extremely strong depending on when and how you use it. If an ally with Skill Mirror uses a Self attack, said self attack will be cast twice; this is very strong on self extra turn attacks. For a very, very specific use of this move, you can pair Ragnarok with Osteoclast, have Ragnarok use this attack and have Osteoclast use Bone Shell. The move will be reflected back on Osteoclast, giving him two extra turns instead of one. This allows him to use Bone Power followed by Skeleton Army to deal massive damage to the enemy team, possibly even wipe them out. Or, you can simply run this to reflect deny moves or extra turn attacks back at enemies, which will make them hesitate in using those moves. Usage depends greatly on both ally and enemy team.

-Demon's Flames
This move is very powerful against deniers who don't have extra turn skills. Total Blind denies them while the trait disable allows the application of further effects. You can disable opposing Anticipation monsters with this move, which allows your extra turn users to safely operate, or remove Control Immunity, Artifact or Hardened forms from targets to make them easier to deny/apply debuffs to. Usage depends on enemy team, though this is a bit more universally useful than other moves with this requirement.

-Grandmaster's Wisdom
Since Ragnarok is at an awesome Speed tier, he can use this move to prevent the fast monster on the enemy team, allowing your team to operate better. The Curse it applies is nice as well, which makes this attack great against stally monsters too. However, usage of this attack requires knowledge of the monster you're targeting; use it if you're sure the monster doesn't have a deadly 0 CD move. Usage depends on enemy team, since he has other single target deny attacks.

-No More Failures
One of the most powerful turn transfer skills in the game, this almost completely refreshes the target ally before giving it the turn. Fast versions of Ragnarok can just use this as a regular turn transfer whereas stall-friendly Ragnarok can use it to refresh a wall that's hard to take down, like Dunn Ra. It is very handy to have a reset button like this on hand, so this should be one of the moves to try to get into your movesets most of the time.

-Queen's Call
This will replace They Are Weak! when you get access to it. It is Ragnarok's first true deny skill, and it's even able to get through buff-induced immunities to Freeze, like Evasion. You can use it for the Mega Freeze, which is very powerful on its own, but you can also use it as a straight up buff removal. Usage depends somewhat on the enemy team, since he has other single target deny attacks.

-Undead's Ritual
Revival skills are strong, but Ragnarok's revival skill gives the target Double Damage and transfers the turn to it. A situational attack, but if you feel like your allies are going to die in a battle, run this skill to turn the tables around. Usage depends greatly on ally and enemy team.

-Dwarf's Stomp
Ragnarok's other true deny skill is indeed a very powerful one: it Mega Stuns the target, but algo gives all allies Control Immunity. This makes it an obvious choice on fast variants, to deny the enemy's fast monster while keeping your team safe from most forms of denial. This also happens to be his most powerful attack in terms of Base Power, with a solid 60. If the enemy is not using an ordinary control skill and/or is immune to Stun (the first part is the important one), you may consider Queen's Call or one of the other deny moves he has instead.

Now I will list some example movesets for each Rank, explaining what they all aim to do. Since I analysed all of the moves, I won't go over them too much.

Builds:

Rank0:
(Stall-Friendly)
Moves:
-Mother Nature's Energy
-Archangel's Blessing
-Blacksmith's Masterpiece
-Grandmaster's Wisdom
Runes: 3 Speed/3 Team Speed
Mutant Rune options: Speed&Life/Team Speed&Team Life

Build Explanation:
A set which aims to keep allies alive for as long as possible, while reflecting attacks with Blacksmith's Masterpiece. You should use monsters who benefit from staying alive for long periods of time with this set, like Ouros and Cryotan. Demon's Flames is an alternative in the last moveslot if your team likes to use extra turns and the enemy has an Anticipation monster, but it's situational.
3 Speed makes this Ragnarok as fast as possible to use Grandmaster's Wisdom or Blacksmith's Masterpiece. Life Mutations are the most viable ones.

(Denier Killer)
Moves:
-Barbarian's Warcry
-Death's Door
-Archangel's Blessing
-Grandmaster's Wisdom/Demon's Flames
Runes: 3 Speed/1 Strength 1 Speed 1 Life
Mutant Rune options: Speed&Life

Build Explanation:
This is a very specific and situational build, but it can be successful if used right. If the enemy denier is a Water monster, like Warmaster Thalassa, Barbarian's Warcry's power will increase against them, and if they're not running Life Runes, they will almost always be in Death Door range. Archangel's Blessing is always there for its usefulness. Grandmaster's Wisdom or Demon's Flames takes the last slot, as the closest thing to killing deniers is denying them.
3 Speed allows Ragnarok to kill Water deniers before they move, and Life Mutations are the only viable ones.

(Rainbow Ragnarok)
Build is the same as the Denier Killer, except it runs 1 Speed 1 Strength 1 Life/2 Speed 1 Strength for Runes
Mutant Rune options: Speed&Life, Strength&Life, Life&Speed

Build Explanation:
This Ragnarok has the same moves as Denier Killer, but it can kill monsters that aren't of the Water element as well, thanks to the 1 Strength Rune. Game plan doesn't change: use Barbarian's Warcry, followed by Death Door to kill the target. The moves here do not change across Ranks, so it won't be mentioned again.
The given Rune spread gives Ragnarok enough Strength and Speed to operate, whereas the 1 Life Rune increases survivability. Alternatively, you can give him 2 Speed 1 Strength to kill some faster monsters. Life Mutations are the only viable ones, and the Speed Mutation on the Life Rune gives him a bit more Speed since he needs all the Speed he can get on that set.

Rank1: Ragnarok learns "No More Failures" at this level.
(Stall-Friendly)
Moves:
-Mother Nature's Energy
-Archangel's Blessing
-Blacksmith's Masterpiece
-No More Failures
Runes and Mutant Runes are the same as given above.

Build Explanation:
No More Failures is extremely helpful to refresh a stally monster. Not only this, but it can also be combined with Blacksmith's Masterpiece: if the target has Skill Mirror, No More Failures will be reflected back at Ragnarok, not only totally healing him, but also giving him an extra turn alongside the target. This build is also very useful as just a fast turn transferrer, so having a monster to benefit from this aspect is useful.

(General Utility)
Moves:
-No More Failures
-Archangel's Blessing
-Blacksmith's Masterpiece/They Are Weak!
-Demon's Flames/They Are Weak!
Runes: 3 Speed
Mutant Rune options: Speed&Life

Build Explanation:
As the name implies, this Ragnarok aims to have as many powerful buffs and debuffs as possible to ruin the enemy's plans. If either one of Blacksmith's Masterpiece or Demon's Flames are not too useful in a given fight, you can replace one with They Are Weak! to have a spammable buff removal.
3 Speed and Speed&Life are there to make Ragnarok as fast as possible so that he can function as an effective turn transferrer and get more turns to use attacks as often as possible.

Rank2: Ragnarok learns "Queen's Call" at this level.
(Denier Killer)
Moves:
-Barbarian's Warcry
-Death's Door
-Archangel's Blessing
-Queen's Call/Grandmaster's Wisdom/Demon's Flames
Runes and Mutant Runes are the same as given above.

Build Explanation:
Same game plan, with Queen's Call added as one of the possible deny options.

General Utility will see They Are Weak! replaced by Queen's Call, as while it's not spammable, the difference in power levels is too great to ignore.

Rank3: Ragnarok learns "Undead's Ritual" at this level.
(Stall-Friendly)
Moves:
-Mother Nature's Energy/Undead's Ritual
-Archangel's Blessing
-Blacksmith's Masterpiece/Undead's Ritual
-No More Failures
Runes and Mutant Runes are the same as given above.

Build Explanation:
Undead's Ritual is necessary on this set, since revival effects are very effective in stall. Use it over either Mother Nature's Energy or Blacksmith's Masterpiece depending on whether Damage or Skill Mirror is more useful in the given situation.

(General Utility)
Moves:
-No More Failures
-Archangel's Blessing
-Blacksmith's Masterpiece/Queen's Call/Undead's Ritual
-Demon's Flames/Queen's Call/Undead's Ritual
Runes and Mutant Runes are the same as given above.

Build Explanation: Revival is situational, but powerful. As such, General Utility can find a spot in its moveset for this attack. As always, pick the last 2 moveslots depending on the enemy's team comp.

Rank4: Ragnarok learns "Dwarf's Stomp" at this level.
(Denier Killer)
Moves:
-Barbarian's Warcry
-Death's Door
-Archangel's Blessing
-Queen's Call/Grandmaster's Wisdom/Demon's Flames/Dwarf's Stomp
Runes and Mutant Runes are the same as given above.

Build Explanation:
Same game plan, with Dwarf's Stomp added as one of the possible deny options.

(General Utility)
Moves:
-No More Failures
-Archangel's Blessing
-Blacksmith's Masterpiece/Queen's Call/Undead's Ritual/Dwarf's Stomp
-Demon's Flames/Queen's Call/Undead's Ritual/Dwarf's Stomp
Runes and Mutant Runes are the same as given above.

Build Explanation:
Dwarf's Stomp offers Control Immunity to the team, which is invaluable, in addition to Mega Stunning the target. Decide on the last 2 slots depending on the enemy team.

These movesets are just the tip of the iceberg regarding what you can do with Ragnarok. You can get more creative and use the interesting interactions between buffs and monsters to create more specific, yet sometimes more powerful, movesets.

Teammate Options: You can use Ragnarok in any team you want. If I were to list each specific interaction his teammates can abuse, we would be here forever, so I will not do this.

Countering Warmaster Ragnarok: Your best bet is just outspeeding and denying him, since you may never truly know what he's up to. Depending on his SC, even that may not be possible. Another option is using a Taunt monster; Koralle Brutalis is vicious against Ragnarok since Ragnarok has no AOE attacks, meaning he will only be able to hit Brutalis with most of his attacks. However, this will not prevent things like Blacksmith's Masterpiece or Dwarf's Stomp from buffing his team.