If you're talking car subs, then...
My younger brother was a car audio NUT...he was blind and had some other serious medical problems, so it all came down to sound. He died a short while ago, and before he went he bought a pair of 12" Kicker CompVR's, 2 Ohm version, and a sealed enclosure. Oh man, i hooked those bad boys up recently (in honor of my bro) into my car with it's 600Watt Kenwood Amp...the speakers come with internal crossover wiring, dropping the boys to 1 Ohm...and HOLY CRAP!!! Loudest, cleasnest bass I've ever heard! If you go that route, you will NOT be let down (but you must buy a good amp, at least 500Watts)...
Kicker CompVR link