What is the best subwoofer you have ever heard?

I have demoed the JL 110, and they will be my next purchase. They sound great!

See if you can demo some JL Audio and Velodyne subs. F113's, Gotham, DD18 plus or 1812. Very impressive.

If I were you, audition two SVS SB13 ultra subs - 45 day trial. I have a PB13 Ultra(best sub I've owned and heard) and i'm going to try a SB13 Ultra, decide which I like better , and get another of choice. Depending on your room size, 2 SB13 Ultras cost you the price of one JL audio F113, 4 will cost you the price of a Paradigm Sub 2 in price. If cost is no object, I'd get 2 Paradigm Sub 2's and calibrate them and be done.......so done.......

The best subs that I've actually heard in a demo are the JL Fathom and the Revel B15. Tight, articulate, fast, and authoritative bass. I liked it so much I got the Revel. The Fathom goes deeper than the Revel and if I had the cash would probably be the only other sub I'd go for. The RELs are the other brand that I would include in that as part of my top three.

I've previously owned Martin Logan, REL, and also auditioned (but not owned) Sunfire.

The ones that have been best set up and integrated into the rest of the system and the room.