Which material does sound travel through best?

Dense solids.

Related Information:

Sound waves travel mechanically, by transferring their energy, molecule by molecule, through a transfer medium. So, the more dense the transfer medium (molecules closer to one another), the faster sound energy is transferred.

Different perspective:

Sound travels by vibrating the particles of the medium through which it travels. So sound travels best in steel as the particles are tightly packed, through wood or water but poorly in air, and not at all in a vacuum.