Looking for input: Best material for mid range cone

Whatever floats your boat is the right answer.  I do however like the Youtube video that Vandersteen puts out showing the pistonic nature of their Carbon Fiber cones/drivers they hand build in Cali.  The planers and ribbons can not recreate that and neither can the paper cones in the video (They are from a very expensive, high end brand speaker). 

To me, accuracy is so important as are measurements when they design these speakers.  a great designer can and often will design a speaker on measurements and know what it will sound like.  Then they will listen and tweek accordingly.  They will listen to various materials and some take the cheap route and tweek an off the shelve driver or use an exotic material for sound, but much of the time for marketing.  I didn't listen to Vandersteen's until they started to use carbon fiber technology and to my ear it's made the most significant difference in speaker sound when utilized correctly, than an material I've heard in over 30 years or so.

For those who haven't heard the model 7's or 5's you really should take your favorite recordings and go listen.  I did and ended up switching speakers a few years ago.  There are still many speakers out there that I like, but none that I love like the Vandersteen's.  Getting value at the higher end is rare and I do appreciate that also, but it's still all about the sound.

Implementation, as I said before is KEY....hard to argue that.  

That's why I like my Focal. It's very hard to end up with a lousy speaker if it's designed to exhibit good measurements all around. The buying public seems to de-emphasize measurements far more than designers and professional reviewers. I've grown to highly appreciate the measurements Stereophile does because they're very predictive of what a speaker will sound like. 

Stiff and light with breakup significantly out of the crossover frequency 

simple really

till you try try and build it.

look at energy storage and pistonic motion IF you are not a science denier...

I became a planar guy after I purchased the Martin-Logan Aeon 12 years ago.  Since then, I have owned Magnepan 1.7 and Quad ESLs (I think the model was 2805) .  I still have the Maggies, but I am not using them.  A few months back I purchased a pair of Kipod 2 speakers used at a very nice price.  The Kipod speakers use aluminum speakers machined from blllets of solid aluminum for the midrange and woofer.  While they are very expensive if purchased new, you can expect to pay 25% of the  new cost for a good used pair.  These speakers are incredible in the midrange and the soundstage, tone, etc. are spectacular.  I prefer the sound of the Kipods over any speaker I have ever heard.  Save your pennies and look for a good pair of used Kipods or other YG speakers with aluminum drivers.