It depends on my application but usually something with a good build quality from a reputable company, decent to long throw, low to mid Q, mid to low Vas, low inductance, high motor force, and a plain and simple face approach (ie: Dayton drivers).
If I'm wanting a setup with thick low end, I want a low Fs, mid Q, higherish Vas, long throw, low inductance, and a simple look to them. A linear BL approach would be nice as well as Neil mentioned.