the best sub is pretty much the sub that you like the most. go listen to some that you can afford and when you find the one you like the most that would be your best sub
If you do a blind test with 50 different subwoofers and you end up liking the $25 the best, then that is the best subwoofer under 400 for you. Simple as that. It's a completely subjective question that has no definative answer.