Please provide much more info as to your install, budget and goals outside of just "SQ" because massive SPL leaning sub woofers such as a Fi BTL can sound very good.....
^^ This. I'm also just going to throw the "Fi Q" out there. It's more of a SQL sub but it sounds great. If you plan on using it in a sealed enclosure it needs to have the high QTS option, though
I would go with ssa if you want sq with a good amount of spl also some boston gear is similiar sql. Hertz and focal for just straight up sq. Morel for amazing sq.
These are just the top of the line models from the respective manufacturers. Lesser/cheaper models could also provide good SQ. It all depends on your setup. Once you narrow down to available wattage and amplifier impedance as well as the space for your box, it would be a little easier to decide.