even though I understand your answer (favorite because of the bang-to-buck ratio), is there another that you would consider "best", even if the cost-to-value ratio was a lot higher?
The next choice would be the Eminence HL10a, but I'd never use that outside of a pro-sound application. The MCM 55-2421 is what I have in my own livingroom and recording studio subs, and it has nothing to do with price. I always place a premium on what works, and with that being satisfied I then go for value, not the other way around.
Tuba 18 is going to appear in audioXpress magazine this fall, and after it has been published I'll have the projct available on my website. I only use one for my HT and have never come close to its capabilities. It will put out 115dB without even breaking into a sweat.