Are you avoiding 15" subs for space issues, they do require a much larger box, but a 15" subwoofer will have a definite displacement advantage over a 12" or 10". Simple math will show you the cone area advantage. In SPL that is very important. You have to look for a subwoofer with a high displacement. I think a pair of 12" subwoofers in a tuned ported box would make a good starter system.
Although this has already been discussed, I feel by simply pointing to another thread you miss out on current events. What was the best 6 months ago may not still be the best.
If you provide a price range for what you want, we can make better suggestions for you.
If you want massive displacement for under $200
http://www.stryke.com/AVseries.htmlThe stryke AV would be the right sub for you, no question about it. displacement/$ wise it's the best choice out the right now
If you can spend slot of cash for subs
http://www.adireaudio.com/mobile_audio/drivers/brahma_series.htmThe Brahma is a beast, and has a very good level of sound quality.
In either case you'll need an amplifier, and right now I'm recommending the JBL 600.1 and 1200.1 amplifiers since they are very inexpensive and offer alot of power.
But like I said, provide a dollar figure, how much space you are giving up, and we can better assist you.