If I was going for the best, I'd get the Accuton D20-6. The diaphram is made of diamond. Claimed frequency response to 100K. Avalon uses it in the latest version of their Eidolon speaker (the Eidolon Diamond). They are available from Madisound.
Oh, there is one little problem. They sell for $2800.00 EACH. :-(
Oh, there is one little problem. They sell for $2800.00 EACH. :-(