Best subwoofer?

Perhaps, you should have stated some constraints with respect to your question, such as size weight and cost. This is from the perspective that the best subs out there are very large (as a minimum, think 3' by 4' by 5'), generally well braced and massive (300-500# per side) and very expensive ($10-20k per sub). That is not to say that there are not good choices at other price points and with smaller size. Of the subs mentioned, I would recommend either the larger Audio Physics subs or the Martin Logan Descents. I have had very positive experiences with both, albeit in other audiophiles systems. I have also heard good things about the digital Velodyne self equalizing units but have no personal experience.

The best that I have heard are the Cabasse Saturn 55 Active Subwoofers which are hideously expensive. A 21" concave Cabasse woofer (built in their factory) in a 37" by 33" by 28" box with a 1200 watt amp, parametric EQ and low pass filter, capable of generating 135.5 db at 1 meter. 350# per side, massive and inert. I had used these in the past with a pair of Acapella Campanile speakers with mixed results (the bottom octave was fabulous but they never mated perfectly with the Campaniles. Two weeks ago, I set up a pair of Acapella Triolons in my system which use a horn loaded 12" woofer to cover the range from 170-700 hz rather than multiple 10" SEAS drivers in an acoustic suspension enclosure). For whatever reason, the mating is seamless with the subs crossed at 40 hz. Normally you do not realize that these are in the system, except for an increase in low level ambient information and air and a concommitant increase in the volume of the sound stage, particularly with respect to depth.
However, when low bass exists, the response can be awsome. Are these really the best subs? Who really knows? What I do know is that they are by quite a margin the best that I have heard when used properly in the right system.