Depends on the type of suspension you have. If it is the now typical solid (or mildly isolated) suspension then hard maple butcher block makes an excellent base. Other woods may work too, but hard maple is the most recommended. Using harder materials like stone, glass, or maybe acrylic provides less damping and can negatively impact the sound. There are of course folks who will insist the harder materials work fine, but you takes your chances.... Now if your turntable has a spring suspension you may have more choices, the springs help isolate the table from the shelf material, though I would still suggest maple.
