What is the best type of sound insulation?
See the links for several options. Others can be found with a Google search (floor+soundproof). The degree of “soundproofing” will depend on thickness and material chosen. Note, however, that all of these materials will only work right if there are no nails or screws from the top layer (finish floor) into the subfloor to creat bridging (or coupling of the two surfaces). The sounddeadening works by decoupling the vibration (noise) of the finish floor from the subfloor and joists. This means you should be thinking of a carpet or floating laminate or glue-down hardwood mounted onto a second plywood subfloor layer ($$) above the sound deadening material.
