There are many good subs out there, most of which I have little or no experience with, but I can toss out a few models or manufacturers that I see come up fairly regularly. On the internet-direct side of things, you have the Outlaw Audio LFM-1 EX, HSU Research VTF-2 or VTF-3, AV123's MFW-15 (which has experienced back-order issues until very recently), and an assortment of offerings from SVS. I've also seen a lot of talk about the JL Audio Fathom units lately.

I've probably left out several good subs for each one I've listed above, but it may help give you some ideas. Others here can probably offer some other ideas - might even be useful to skim through posts in the "Your System Saloon" area to see what others are using.