The term "Ultimate" yields itself to interpretation, particularly insofar as tone is concerned. What's pleasing to.my decrepit ears might be offensive to others, and so it goes.
With that being said, my several years on this forum have yielded the following observations:
I've categorized three different levels of cabinetry (with exceptions)
1. Top tier "boutique" cabs
Barefaced
AudioKineses
Greenboy designs
Mostly full-range, high power handling, light weight, superbly designed, engineered, and produced. Alex at Barefaced, for instance, makes 3 different single 12's: the Super Midget; Super Compact; and Big Baby II, with various capabilities and form factors.
2. Top-of-the-line mass production cabinetry
Bergantino
Epifani
Mesa
Aguilar
Again, great engineering and consistent quality. Excellent materials, sometimes even better than the smaller boutique guys. Great customer support (if one isn't a tool about things)
3. Regular production stuff
As noted, there are some products which fall somewhere in between, stuff like BagEnd comes to mind*, and others might consider them to fit into the first tier group. I've never taken the opportunity to play through one so I'm speaking somewhat through ignorance.
These are more general impressions rather than scientific analysis so take it for what it's worth.
I'd like to have one of Duke's cabs one day. I do have a 1-12 Berg which is VERY nice but being equipped with a ceramic magnet, it's a bit heavy.
I seem to.recall that someone did a boutique shoot-out a year or so ago. Might be worth checking out to at least provide general brand knowledge if nothing else. There's a Talkbass truism that I've seen a time or two: Cost, weight, sound...pick 2.
That's just my take on this fwiw.
Oh yeah, and Mark S. smh
*edit: Schroeder and Revsound might fall in to this slot, and as RColie points out, Genzler appears to be a top-tier large production outfit as well.