A few things that matter for a sub:
What space considerations you have. Some subs will want a TON of airspace to play properly. The sub's T/S parameters will tell the story and you can model a correct box size based on calculations.
Frequency response desired (i.e., does it need to reach down low due to cabin gain issues in your car, or does it need to have upper frequency extension to blend into a weak front stage?)
What alignment do you want? Do you want flat, smooth response, or do you want something to win SPL shows and scare small children (but sound non-musical)
How much REAL power do you have to provide it? Efficiency can be very important, as well as what impedance (ohm rating) you are working with. You don't want a single 12" dual 2 ohm coil sub when you are going to drive it with a 2ohm stable amp, for instance.
How critical are you of distortion?
And now, here's a few brands that should be on your radar.
Phoenix Gold RSdc.
Fi Audio SSD and/or Q line
Sundown Audio
Soundsplinter
Eclipse's older lineup
Stereo Integrity (AWESOME, my next upgrade)
Critical Mass
Aura
Dayton (check
http://www.partsexpress.com)
Really though, if you aren't changing your box (and please God tell me you are not going to stuff a new sub into a random ported box), your options will narrow considerably. You can take the box you have decided, measure the inner airspace, and quickly eliminate many of these subs right out of contention due to incompatibility.
You are looking for a Qtc of .707 or so using calculations. This will yield the most "area under the curve" as it were, for the best, smoothest, flattest, and most natural sounding bass response. Cabin gain will boost this a bit but hopefully it will boost right where the sub starts to roll off in response (Assuming sealed box.)
opcorn: 
