JL Audio 10W0v3-4 10" W0v3-Series 4-Ohm Car Subwoofer
You won't be disappointed.  I was looking for the best subs for the lowest price possible and JL audio didn't disappoint.  I have these in the smallest 10&#34; box I've ever seen (.3 cubic ft I think) and they still thump in my pickup truck.  I can only imagine what they could do in a proper spec enclosure.  Best 10's ive ever owned for the price.