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in general, including on ceilings, thicker absorbers will give you absorption to lower frequencies. This effect is enhanced if you space the absorber out from the wall a bit; a spacing equal to the thickness of the panel is a good rule of thumb, ie, 4" panel 4" below ceiling, 8" panel 8" below ceiling, etc etc.
Surprisingly, a 2" panel 2" from the ceiling performs almost as well as a 4" panel flush with the ceiling, despite it having half the mass.
Rigid fiberglass, rockwool, acoustic cotton all work well. Use what you have on hand, whatever you are using for your bass traps/broadband absorbers is fine. If you have bass trapping in all corners you can get away with 2" panels spaced 2" from the wall.
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