Skill level necessary to clear HW MSQ content

[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.