there isnt really a "best" brand or "best" subwooofer... it is more dependent on what you are going to do with it, how much you want to spend, and most of all the install.
there are of course some that are inherintly better than others in some aspects, but not any way to say the best.
think about how much money you want to spend, how much trunk/storage space you want to sacrifice, whether you want SPL or SQ(loudness or sounding good), and how much power you are going to have.
think about those things, and let that dictate your decision. hang around the forum for a while, do some research, and then you can make an informed decision. it will be worth the wait and work, believe me.