Strictly speaking, that is, picking ONE sub (as opposed to, say, dual PB-13 Ultras), I would nominate the Seaton Submersive. Which is about $2,125 delivered, plus a really good EQ like the SVS EQ-1.
Then too there is a new sub coming from JTR, a revised Captivator that is supposed to perform as well as the original model, but for half the price, and size. Target price is $998, plus an amp and equalization.