Lots of good choices mentioned. How much power will you have and ported or sealed? If ported how big of a box can you do?
RL-P has excellent SQ and will get pretty loud and can go ported in 3 cubes. Hit the lows nicely.
Q will get louder than the RL-P, but SQ isn't quite as good. Just not as tight/accurate sounding at times. 3.5 cubes works fine.
HD315 will get very loud with enough power and if tuned to 35Hz will have very good SQ and hit low. I ran mine in 4.5 cubes, but it'll go in smaller boxes. Not quite the SQ of the RL-P, but much louder than the Q in the applications I've tried.
HD gets my vote as it's a great daily driver sub and with the new ones having some improvements and being cheaper I think it's hard to pass up if you have the power & space to make it happy...