Looking for input: Best material for mid range cone

Where do you guys come up with these blanket claims? "Polypropylene not so good above 1Khz." Based on what? My speakers have a 7" plastic cone and play up to 3kHz with hardly a lick of distortion. They play much cleaner than numerous other speakers I've heard with exotic cones. There's good and bad examples of almost every speaker diaphragm material. I've heard aramid cones that sounded like complete garbage, and some that sound great, the same for paper, plastic, titanium, and aluminum. I've owned speakers with plastic tweeters (oh no! Over 1khz!) that blew away some well regarded silk domes.

To my next point: size matters! Any loudspeaker engineer worth his salt can tell you that a material's performance can heavily depend on the size of the diaphragm, the frequency range it's asked to cover, and the SPLs it's asked to play. It's true that large plastic cones CAN struggle with higher freqs, while paper cones exhibit their best virtues in the larger sizes. I haven't heard many good sounding paper tweeters, however, like with most materials, I'm sure there's exceptions out there.

OP, 
It really comes down to personal preference and the quality of design. Speaker sound is the most subjective aspect of a system. One person's euphoric midrange will make another's ears bleed.

I have owned rega rs5, which utilize paper cone for both midrange (5") and woofers (7").  The midrange on these speakers was outstanding - natural and transparent, it is really their big selling point.  I have never got the same with kevlar (B&Ws at twice the price, and Wharfedale at equivalent price).
The new line rx also employs doped paper cones... Haven't heard them; I am not sure how they compare to the rs.

It’s so interesting to read the varying thoughts on this topic. My issue is not the frequency range that poly can reproduce, but what it sounds like while doing so.

The Tannoy Precision bookshelfs I found so enjoyable have the tweeter crossed over at 1600 Hz, which is much lower than the Sierras. Perhaps that also has something to do with it?

I suppose there are no absolutes in speaker design, except that opinions will differ!

Poly cones can have huge variances in tonal qualities, just as other materials can.

The Poly cones of my Epos Epics sound different from those of my Spendors, which sound different from those of Harbeth. Polycarbonate has derivatives, similar to metal alloys. Then there's the effect of the basket material, the basket's frame design, the cone surround material, implementation of dust cap or phase plug, the spider, and the voicecoil design. All have an effect on the driver's sound. 

A plastic driver with bullet dust cap will have different tonal characteristics from one with a bullet phase plug. 

It's my opinion that the real test of  quality drivers (and components in general) is whether you can listen to them for long periods without fatigue, get lost in the music, and forget about the system. Often times, people automatically perceive "different" as being superior. Though, if they listen to these excitingingly different speakers for long periods, fatigue sets in, or they end up returning to their previous pair. You might find the Tannoys to be superior now, but consider how much time you've spent with them since swapping out the Sierras, and consider how you feel after a 2 hour plus listening session. You might be surprised if you swap the Sierras back in after a few weeks, then again, maybe not.

Generalizations in regard to cone material are just that: generalizations.

Wow, interesting thread with no right answers.  If you want to talk measurements, you need a driver (cone is only part of a driver and then it's the crossover and other materials that make up a great speaker.  The mids shouldn't stand out. If they do, then you lose coherency, which isn't what high end audio is about.  

A great mid driver needs to be fast.  This is the one advantage that the ribbons and planers seem to have, however to properly reproduce a square wave (a great speaker must have excellent measurements in addition to sounding great), you have to have a driver that will be as pistonic as you can get.  The cone needs to be as light weight as possible AND as strong as possible to display the speed of a ribbon or planer (or as close as a driver will come).  

Only a few materials have been mentioned in this thread.  Right now, the best (again, subjective, but being used by Vandersteen with it's balsa wood cone wrapped by a specific carbon fiber that was chosen for it's sound quality over many listening sessions) seem to be cones made of carbon fiber.  Some companies are using a carbon nano composite and they are also fast and strong, but I've yet to hear one that is piston in nature and it doesn't produce a musical tone to my ears, but others will disagree.  

I have yet to hear as haunting a mid as the Vandersteen 7's and this is one reason I'm a fanboy.  Can't help it and in years to come if someone else does it better, then I'll be a fanboy of their speakers.  What so many don't realize is that a great mid bass speaker MUST have a very very fast mid.  The midrange truly is 90% of what we are hearing and the problem with so many on the market is distortion.  

I've only liked a few ribbon speakers.  I do like some planers too.  The key to any system is matching components also.  

See how there is no correct answer per say.  Maybe in the future, we will get other materials that will be made in 3D printers that will crush what's currently on the market, but for now, my money is on the Vandersteen Carbon Fiber cones for both midrange and tweeters.  JMHO