Best Loudspeakers for Rich Timbre?

Yes aluminium and ceramic are nearly perfectly rigid and pistonic but they Rrriiiiinnnnnnggggg like a bell and are terrible materials for driver cones. The Rrrrrriiiiiiiiiiiinnnnnnnnnnnnngggggggggiiiiiiiiiiiiiiiiinnnnnnnnnnnnnngggggggggg is what I hear from these designs. Terrible waterfall plots in most cases. Magnesium is better. CF is pretty good. What Richard uses will also be good - carbon fiber and balsa - both these materials will dissipate internal energy.

The second video on the page I linked above shows that the Harbeth radial cone supports a 4.5 Kg weight - this is pretty good rigidity!!

A good driver balances rigidity with intrinsic internal damping. The use of a large diameter voice coil can also make a huge difference instead of the typical 1 or 1 1/2 inch consumer audio woofer voice coil (which offers little support to the diaphragm and places a lot of stress at the apex of the cone)

That is why the cones should be sandwiched, creating a constrained layer damping mechanism. See Magico and Zellaton. Focal does that as well, but uses fiberglass, somewhat less effective.
Magico uses 3" and 5" voice coil, btw.

My original Legacy Focus speakers use cheap paper woofers (3 per speaker) and kevlar mid-ranges (2 per speaker).  Legacy upgraded the cones and doubled the magnets in these speakers as Focus HD and SE.  Neither of the two newer speakers sound as musical as my old fashion, cheap cone material speakers.  How much better would my Focus speakers sound with Harbeth cone drivers?  Apparently, a lot better.   But to date, my Focus speakers are overall more enjoyable than Harbeth 40.2s I've heard four or five times (sounding very nice too).

@sciencecop 

 I agree with you about cone design. The ideal cone is perfectly rigid and perfectly and completely damped.

I have to say, if you mean truth of timbre, or closer to real sounding in the mids, the new Revel f 228 be is outstanding. Check out the technology and measured response of these speakers,  truly impressive. The best I have had in my home, and will put far more expensive speakers to shame. They are something special when set up properly.