For me, it's the B&S F tuba with the fully 16-21mm graduated bore, currently the Model 3100, but known in the past as the "Symphonie".
https://www.b-and-s.com/en/instruments/f-tubas/3100-2/
Like the 188, the current version is not sprinkled with the same fairy dust as the earlier versions, and for the same reason: The early versions were far more hand-made than the current versions.
Also like the 188, it's a little smaller than current orchestra professionals (and those who wish to emulate them) presently prefer. But, again like the 188, it's larger than it looks, and didn't seem to be a problem for notable past performers. And both were considered "large" compared to "regular" instruments when they were first offered.
More similar attributes: Easy intonation tendencies, a clear voice in a large ensemble, plenty loud even for big works, and quirks that are much easier to manage than those who don't play them realize. And though both are offered in their catalogs, one does not see them very often at shows (at least back when we had shows), or in stock at stores.
Since the early 80's, B&S has increased the bore of their "standard" F tubas, seemingly to make them a bit more contrabass-like. In my view, it makes them a little more hollow-sounding and doesn't really relieve their quirks fully enough to be worth losing that sweet quality.
Rick "can't blow the low C like a contrabass, and the effect of the 'dent' is not transferrable" Denney