there is no single objective answer to that question, as there are many factors that will affect the result. vehicle, power applied, enclosure type and size, the rest of the system, the charging system available for the amplifiers, your listening tastes, system goals (SPL? SQ?) and so on and so forth.
Really, there's no simple way to answer that question. Budget plays a huge factor as well. You can spend anywhere from $40 to over $4,000 for a single subwoofer. Your checking account plays a big factor in performance and choices.
i'm gonna have to agree with glasswolf on this one...theres no one best sub, and he's right, your checking account plays a huge factore in the system you get, my first system was cruddy...mediocorly bad at best, but you upgrade, and new prodcuts come out, its an expensive hobby, if theres one thing to live by in car audio...