This list is a little hard to judge on considering that we have SQ class subs vs. SPL Dedicated subs. . If i had to give my opinion of each sub though. . .it would go in this order
First off i would take a Team Smoke 18" over any of these subs. . .
DD9515 - Sub can take torture and it sounds great all the way around. Can easily produce high numbers of SPL and can be used in SQ applications as well. I would rather have a Treo CSX then this sub though
MMATS Juggy - My personal favorite SPL sub. I used to own 4 15's and it would give me a headache to sit in the car and turn these beasts up. . . Alot of people dont know that the Bostwick DCL can put up just as high of numbers at the Juggies can.
15'' Brahma - For the price you pay, you get a very reliable sub, it has a very high Xmax can can handle the cone travel without any loss of SQ, has a flatter BL curve then that overpriced peice of shit W7, and sounds good in small or large enclosures. Definately a bad ass bang for buck sub.
15'' XXX - I see this sub pretty much equal to the Brahma. Uses the same technology. Tends to be a little Boomier then the Brahma, but the Brahma has slightly more SQ. If these two subs were the same price, i would take a XXX over a brahma anyday.
18'' Cerwin vega stroker <---18'' again - Still a top class competitors sub. Sounds like ass IMO, but has the ability to put up high numbers. . . they are just designed off old technology
15'' kicker L7 - Gets reasonably loud in small enclosure. . . not a kicker fan though. . .i would never but one. If someone gave me one. . . i would sell it and buy a real sub
15'' power hx2 - Not a RF fan either. These subs get loud but sound like ass. . build quality for RF has went downhill and i just dont trust the company anymore.
Other Subs that deserve mention. . .
Elemental Designs A Series
Bostwick DCL
OZ Audio Matrix Power Series
Shocker Audio
Treo
Magnum Audio
Onyx Audio
Blueprint (out of buisness

)
Sound Stream Exact, SPL 160 or Mule (i love these old school Subs)
Image Dynamics ID Max