If you want sound quality hands down, first you need to take some time and determine what you really need for your vehicle. Sound quality really depends much more on the overall system than the woofer alone. Measure the transfer function of the vehicle (cabin gain) first and then you can see what you really need in terms of woofer and enclosure to work best with the vehicle. Then you can weigh the tradeoffs of various woofers, enclosure types and sizes, etc.
In terms of woofers alone look at the design and build of the woofer. What has been done to eliminate mechanical distortion, cone breakup, non-linear distortion from the motor, remove heat from the coil, etc. Here's a good overview of the Lambda motor and the steps taken to lower distortion, inductance, and power compression.
http://www.aespeakers.com/Lambda001-1.php
John