Rockford Fosgate
The entry level 'punch' series of fosgate doesn't really compare to the better ranges of rockford sub. This said, even the expensive rockford stuff doesn't compare to some of its competitors.
I would love to make a point here, your sound is only as good as your source. I looked at many options and the greatest improvement i found was to purchase a nakamichi head unit. The CD player has a 24 bit DAC, whilst neither alpine, sony or pioneer extend further than a 1 bit DAC. Plus 4 5V pre-outs, 'nuff said'