I'm currently using 1/2 thick glass slabs with either BDR cones or Vibrapods. I'm very satisfied with the combination, however, nothing is perfect and I don't believe any single material works in every situation. Consider this -- the best shelf material may be no material at all. Instead of placing components on a shelf, have the equipment suspended by small diameter cables. The energy storage capabilites of the suspending cables is practically zero, hence they should have negligible negative effect on the sound of the component. This method will work only if the suspended component has an extremely rigid case. A few years back a product like this was manufactured (can't remember by who), but I never had any first hand experience with it, but the concept does sound intriguing.
Shelf Material
I have tried so many different shelf materials, and some are better than others, but I feel like I am just spraying bullets that always miss the bulls-eye. So far, I cannot live with the brightness of glass, the ringing of marble or granite, the sluggishness of acrylic, the muddiness of mdf etc. Light and rigid seems better than heavy and dense - in that I can live with the downsides more easily. I use heavily constructed welded steel racks - spiked to the floor and upward spikes supporting the shelves - and I reckon this is right. I like the way bladder products get rid of the resonances that plague shelves, but find that the way they slow down the pace of the music is hard to accept. Does anyone have some answers on this?