What's the best? Depends. What are your wants, needs and desires? What is room size? Any size restrictions for the future sub?
$250 may not be a problem whereas $2,500 would be or you may have a no holds barred budget of $25,000.
Too many unknowns at this point to make an informed specific recommendation. However, various companies can be recommended. I would look at HSU, Rythmik Audio, Power Sounds Audio, SVS, Chase Home Theater and Seaton just to name a few.