I REALLY like the Elemental Designs subs. Car audio nuts LOVE them... they are damn sexy and not too pricey. Also look at JL Audio, IDQ, and Resonant Engineering. There are a lot of nice subs out there it's hard to recommend just one.
Oh... and to echo the above, you can make 2 10's sound REALLY good... most of their potential is in the box!!! It needs to be configured and constructed, again, for the potential of the subs you purchase.