The fastest speed that sound can travel is in steel. Sound waves are mechanical waves, which means that they need a material to travel through. They travel the fastest in solids because the material is more tightly packed, making the vibrations travel faster than in liquids or gases.
Steel (Solid): ~5,960 m/s (meter per sec.)
Water (Liquid): ~1,480 m/s (meter per sec.)
Air (Gas): ~343 m/s (meter per sec.)
Vacuum: 0 m/s (Sound cannot travel through a vacuum).