The four most basic layers of Earth are (from center outwards):
Inner solid core of iron-nickel.
Outer liquid Fe-Ni core.
Mantle made up of Mg rich silicate minerals.
Crust made up of silicate minerals rich in Si and Al.

The densest layer is the solid iron(Fe) nickel(Ni0) inner core. The density of the stuff there is about 15 times the density of water at room temperature.