I'm not asking just to fantasize, I am asking because I want to learn more about what makes drivers great, so please try not to throw out impossible schemes like the pole piece should be made of unobtainium or something fantastical like that. I am talking about like what if someone who was filthy rich wanted to make the best conventional subwoofer driver in the world, maybe an 18" or 21" and cost wasn't a consideration.
If money was no object:
dual 15 inch opposed,
50 liter box total
10-100Hz @ 88dB @ 1W
If physics was no object:
dual ? inch opposed,
40 liter box total
1-100Hz @ 95dB @ 1W
What works great in one box does not work as well in another.
So when you say "best"-best for what alignment?
It is like asking what is your idea car? Do you need to carry 6 kids around-or haul lumber-or do you want good gas mileage or 0-60mph or how about cornering -r or going in snow?
There are different needs-and one size does not fit all.
So FIRST you have to determine what is the specific need. Such as how large of a box and what sort of output level are your expecting. How low does it need to go?
Loudspeaker design is all about compromise-so you have to determine the intended goal first-not just "best".
Since you chose your compromise (SPL) then it is very easy to configure a driver:
in a sealed box of ANY size from 10 liters to IB.
B&W 800 Carbon Fiber / Rohacell cone
LMS Ultra Coil (maybe a few more windings)
LMS motor with 3 more slugs added
Two of those, push-pull FI SMD style
28000watt Lab Gruppen Dolby-Lake PLM plate amp
Velodyne servo feedback controller
Dual-opposed, Krell MRS box
now... MAKE IT HAPPEN!