Silicone, glass, and stainless steel can all be sterilized with multiple methods, and are completely non-porous, so those are generally considered the safest materials. There's nothing unsafe about wood, either, although there are less methods for sterilizing it. The same is true of plastic.

If you click on the material listed for any particular toy, there's a breakdown of what cleaning methods can be used, how to store it, what types of lubes are safe for it, etc.