Your choice of the most "inert" cabinets ?

I think the Rockport Lyra unique cabinet design "trumps" them.
I will make a bet that your own measuring tools are not your ears but your eyes.

Maybe the speakers are just as good or better, but only testing will show why.  i.e. it may not be the cabinet

No cabinet is the best cabinet. But that's only one reason why I prefer planars---ESL's, magnetic-planars, ribbons. All but one pair of speaker "cabinets" I do have are actually open baffle frames; the fact that they aren't sealed and are heavily braced reduces the ability of the walls to resonate.

The one pair of sealed cabinets I do have contain subwoofer drivers. I made the enclosure as a box-within-a-box, with a 1/2 space between the two, the cavity filled with no.60 Silica sand. The interior box is cross-braced every 5", and that plus the damping sand results in very "quiet" enclosures.

I agree with bdp24 the best cabinet is none, others can include Bent and Formed Laminations, I have seen fabricated steel used, Celestion used Aerolam , an aluminium honeycombed laminated material. Cast concrete is fun and not to mention an easy favourite , Lead Lined . The sand filled cavity has been used in the UK for transmission line cabinets for years, I love this hobby, all roads can lead to Rome.
MDF is good and practical. Not sure that there is a best. Amusing to see proponents of inert cabinets also supportive of rigid metallic or ceramic drivers. Gee gosh golly - speaker cabinets need to be inert but I guess it is ok for the diaphragms on the drivers to ring like a bell? Slight disconnect or what...
Hmmm, cast concrete sounds intriguing. Wondering why no one is using this approach. Obviously weight is not as much of a concern seeing loudspeakers already out there weighing hundreds of pounds...