Skill level necessary to clear HW MSQ content

[Townowi]

For someone new to EX, which one do you recommend to start with, Garuda, Leviathan, Ultima Weapon or Thornmarch?

I won't start any of them until I can both consistently run these story primals and fix my gear problems in full.

[Legion88]

For someone new to EX, which one do you recommend to start with, Garuda, Leviathan, Ultima Weapon or Thornmarch?

I won't start any of them until I can both consistently run these story primals and fix my gear problems in full.

Listen here sweety.
All the MSQ content you have to do can be completed with the gear you get via the MSQ and in your case by summoning an egi and spam Ruin (minimum requirement, not recommended, you should learn how to play SMN, makes it much easier).
There is nothing in the Ex trials or Coil that will teach you anything you will need for the MSQ.
The only thing you have to learn is to dodge telegraphed AoEs by leaving the orange marked area before it vanishes and the coresponding cast bar finishes.

[Iscah]

For someone new to EX, which one do you recommend to start with, Garuda, Leviathan, Ultima Weapon or Thornmarch?

I won't start any of them until I can both consistently run these story primals and fix my gear problems in full.

...EX primals are not "story primals".

Are you sure you're talking about EX primals? Or MSQ Hard primals?

[Townowi]

I may be willing to settle for one or two extremes per xpac but I'd rather be running the extreme(s) that provides the best gauge for the following xpac. For obvious reasons, synched is a much better gauge than unsynched regardless of the extreme that serves as a gauge.

That said, which ARR extreme(s) is most representative of HW readiness? If applicable, answer healing first and DPS second: pick only extremes of primals that were previously fought in the MSQ.

And I would also like to know which HW and SB extremes are most representative of SB MSQ and ShB MSQ respectively.

Maybe I am wrong but using EX primals as gauges of xpac readiness also imply dungeon readiness for the same xpac that EX is a gauge for. That's why I kept quiet about dungeon readiness.

...EX primals are not "story primals".

Are you sure you're talking about EX primals? Or MSQ Hard primals?

To clarify I really mean that I shouldn't start running EX primals until I get the corresponding MSQ hard primal down consistently and resolve my gear issues.

[MrThinker]

That said, which ARR extreme(s) is most representative of HW readiness? If applicable, answer healing first and DPS second: pick only extremes of primals that were previously fought in the MSQ.

And I would also like to know which HW and SB extremes are most representative of SB MSQ and ShB MSQ respectively.

Maybe I am wrong but using EX primals as gauges of xpac readiness also imply dungeon readiness for the same xpac that EX is a gauge for. That's why I kept quiet about dungeon readiness.

If you are talking about synched... then like has been said before, literally none of the EX trials, across all the expansions, are representative of the difficulty encountered in dungeons and normal trials within the next expansion along. The ARR extreme trials are harder than anything the normal HW story throws at you, the HW extremes are harder than anything the SB story throws at you, and the same holds true for SB Extremes and and the ShB story.

[MistakeNot]

That said, which ARR extreme(s) is most representative of HW readiness?

None of them. Or all of them. The ARR EX-trials are harder than any of the non-EX, non-Savage content in HW.

If you want more practice before starting HW content, then forget all about EX trials.

Instead run, for example, the following standard dungeons and trials from ARR:
Aurum Vale
Keeper of the Lake
The Navel (Hard)
Thornmarch (Hard)

Most of those are required to finish the ARR MSQ anyway, and if you can handle all the mechanics in those - then you have the skills needed for HW.

[MrThinker]

For someone new to EX, which one do you recommend to start with, Garuda, Leviathan, Ultima Weapon or Thornmarch?

I won't start any of them until I can both consistently run these story primals and fix my gear problems in full.

Garuda, as the 1st released Extreme trial, is by definition the easiest, as you can vastly outgear the fight's minimum required item level and thus fail quite a few mechanics before it becomes unrecoverable.

However, as has been said before, the mechanical complexity and punishments of Extreme trials is VASTLY greater than any main-scenario required instanced duty. Getting used to what is required for Extreme will make you realise that most MSQ content is not that difficult in comparison. Not that I am against self-improvement, but if reaching the end of the main-scenario as it is right now is your goal, simply focusing on it will teach you the mechanics you need to recognize.

Also the fact that because they are both 'optional' and 'not in a roulette because the average player wont be able to complete it' (except one, where if people get an extreme trial in that roulette, will most likely abandon ship), q-ing for it directly via duty finder will very likely be an exercise in futility. You will need to set up a Party Finder detailing what you plan to do in the content and hope that some like-minded or helpful individuals will join.

EDIT: Correction, Ultima Weapon is apparently the 1st Extreme.

EDIT2: Also, there are no Extreme trials that are mandatory to complete to progress the story. Garuda, Titan and Ifrit Extreme have a small side story chain associated with them, and some of the info of that chain is later reflected in the story, but is by no means mandatory.