Looking for input: Best material for mid range cone

Actually that's not at all what it being said. What's being said is that many companies are using better quality materials and implementing them properly.  To take a good thread and make a post that sarcastically puts down good companies is silly isn't it?  

Just look at the measurements of these drivers that utilize new materials and driver concepts.  Hard to argue with great measurement AND great sound.  Have you heard the Vandersteen 7 mk2's set up properly?  What about the other speakers that are using up to date technology?  Maybe you have, but to make a blanket statement like that says otherwise.  

Not one person has said anything negative about the other posters love of his speakers, so not sure why you would have a sarcastic comeback.  Interesting...

I am repeating a bit. The textbook ideal material avoids resonance leading to phase distortion.  Another issue is avoiding frequency dependent vibration transmission radially through the cone, a property of solid materials but not air. (ESL is a clever answer as they do not require rigidity to avoid phase/distortion issues). Then the quality of material can depend on the manner in which it fails to handle those objectives, e.g. damping. These failings lead to a multitude of designs to approximate the ideal. A tip-off to complexity is the plethora of blended materials in cones. A lot of other factors come in to play that have been mentioned, and another is cost. I think it is hard to say what is best without more qualification. 

I have a pair of old, $2K speakers with Vifa PP midrange drivers. The midrange is certainly not THE best, but it is great compared to competition at that price including Paradigm.

It is hard for me to accept that aluminum is an "antiquated" material, especially when it is CNC machined to very close tolerances.  The process of making these speakers is very expensive, which is reflected in their price.  The sound is exceptionally distortion free, detailed, and tonally accurate.  The most beautiful speakers I have ever owned.  Think again helomech!! 

Any material used in diaphragm will have properties that require attention to gain optimal operation. Beryllium as an example has very high self damping (stiffness of cone) but doesn't exhibit much internal damping (ability to dissipate excess energy within the material). While the operating range of one will be greater, care to ensure you stay within its operating range for its given size and construction.

I would like to address carbon fiber, since its brought a number of times but aspects of it should be known. There are so many grades of carbon fiber and the epoxies used, construction method employed, and design parameters in that you can come to some very wildly different results with the very same material. You could spend an considerable amount of time in FEA design between differing grades, weaves, shapes, etc. The modulus alone could vary by a factor of five just by simply changing from a low modulus to high modulus fiber. The expense of high modulus fiber is notably more and typically forgoes some tensile strength when modulus increases. But in this application, tensile strength matters little with it being well beyond what is required in nearly all grades. The latest development in this area has been the infusion of graphene in the epoxy, which aids in producing a more uniform carbon fiber sheet. This will further extend how light a driver cone could be made without any loss of stiffness. This still doesn't account for the mass of the voice coil, former, surround, etc. 

So when your looking for the best material, it must include a completed design. Only with this, combined with the desired operating range and crossover implementation could one select the best material within a given cost. I do mention cost since its very easy to gain a marginal improvement for an extreme cost, one which is better spent in other areas of the system.

In terms of completed design, I'm a big fan of composite construction. This includes the Focal W sandwiches, rohacell, carbon fiber laminates, and the cut paper designs used by ScanSpeak (my personal faves).

There's also some newer designs I'm curious about such as the Satori  papyrus / Focal materials using unusually long plant fibers, and Eton also has some interesting ideas about marrying magnesium to resin.