Regular fiberglass insulation or one of the following:
Rigid fiberglass panels, Rockwool, or sheets of thicker poly fill? How thick should those last 3 be if that's what gets used?
IMO, it depends. You talkin ported or sealed.
I have used OC 700 series, Polyfill, and pink fluffy. Sealed I like fiberglass whatever variety you like the performance of, I like OC700 series glued into the walls. IMO for a ported I like the polyfill, rockwool, or jeans material just b/c I'm a little paranoid on fiberglass being shot out the port like a fiberglass cannon. Those fibers are in there vibrating, presumably some are breaking too as they do thier purpose and those particles go somewhere right? If you can keep the polyfill from obstructing the port it can work well medium-loosely packed also.
I haven't used Linacoustic(in a sub) but it seems to be a likely good performer also with approx the same performance as OC700 series. Its also easier to use than fiberglass, less itchy and coated on one side.
I can tell you I prefer the sound of a ported sub with the interior treated from my own testing. Standing waves are not the only enemy of SQ present inside a ported enclosure that need attenuation.
When stuffing sealed I am typically am adjusting the apparent Q.