"Small" and "good subwoofer" do not generally go together well. If it is a question of floor space rather than actual size, so that it can be tall, check out the cylinder subwoofers from SVS. If it has to be small, you will tend to pay more for less performance to get something that is pretty good, and it will never be the best that money can buy, no matter how much it costs, nor will it ever be the best value in terms of price to performance ratio.