In which layer is the pressure the highest?

"Pressure" is the ratio of mass to area. So, for any given area, the one with more mass pressing on it will experience a higher pressure.

As you go more deeply into the atmosphere OR earth itself, you are putting more and more mass over you. For the same unit area therefore, the pressure, (mass/area) increases.

Hence, the lowest layer will always have the highest pressure i.e, inner core.