The most powerful non-caster NPC is the Gladiator, at 15 HD (but only CR 5?).
Maybe you missed the Assassin (only 12 HD, but CR 8) or are you just looking at HD?
Also, Volo's added the champion (22 HD, CR 9) and Warlord (27 HD, CR 12), which was the counter-point to the Archmage, et al.
So Hit Dice seem to be rather arbitrarily assigned, presumably to get the hit point totals desired as opposed to being any real metric of overall power level. I could try to infer relative power level, that is, an Archmage is equal in power to a 12th-level PC, but that doesn't really mean anything. I'm still left with, how many CR 12s exist in the game? And the answer is "as many as a campaign needs".
FWIW, I equate CR 12 to about level 18, or roughly 2/3 of level equal CR. So, a 12th-level PC would be about CR 8.
The Archmage, for instance, is an 18th-level spellcaster but with 99 hp. This is just 7 hp more than a 18th-level wizard would have. However, despite an INT 20, it only has a +9 on spell attacks while the same PC would be +11. This means the NPC's spells are a bit underpowered (easy save DC and lower spell attack), but it has just as many and a bit more HP.
If we look at the Warlord (CR 12 also) and compare that to an 18th-level Fighter, its HP is MUCH HIGHER than an 18th-level Fighter (229 vs. 184!). With its legendary actions it could get up to 5 attacks per round, which is more than the Fighter would get at 3. And although it has some features, it doesn't have quite everything the PC Fighter would have. In a 1-1 match, it would get the same 3 attacks, so I think would be a fairly even fight depending on the Fighter build.
It
would be great if the CR system was designed to have a fairly rough translation system to PC level, and the 2/3 x level is the best I've found for myself.