Fiberglass is good but should be covered in old cotton sheets or pillow slips.

Basalt insulation (Rock wool) is good and should be covered also.

Polyester, not quite as good but can be left uncovered.

Lambs wool is great but expensive.

Those are some of the more commonly known entities but there are lots to experiment with like egg crates (cardboard), linoleum floor tiles, sheet lead, old socks, gel seat covers, gymnasium floor tiles, automotive underlay, cork, immitation cork, carpet underlay...

You get the idea. There is not right material for all applications, some work well in combination with others. If you are speaking of a subwoofer, sometimes nothing is the best as the frequencies are too low for the stuffing to have any effect other then the apparent enlarging of the cabinet.