Which layer of earth is the densest and why is that so?

The densest layer of Earth is the inner core.

Explanation:

The inner core of the Earth, the innermost and warmest part according to seismological studies, is a mainly solid sphere about 1,216 kilometers in radius, or 70% of that of the Moon. It occupies 17% of its volume and represents around 33% of its mass.

It is thought to be an alloy of iron and nickel, and could have a temperature similar to that of the Sun's surface (about 5505 °C).

The existence of an inner core, differentiated from the liquid outer nucleus, was discovered in 1936 by the seismologist Inge Lehmann using observations from seismic waves generated by earthquakes.