A roller coaster's maximum potential energy is located at:

Potential energy is stored energy that is affected by the relative position of various parts of a system. When a spring is compressed or stretched, its potential energy increases. A steel ball has more potential energy when raised above ground than when it falls to Earth.

 The potential energy is given by;

$PE=m\times g\times h$

Where;

PE = Potential energy

m = Mass of the object

g =  Acceleration due to gravity

h =  Height of the object

As a result, the potential energy increases with height. Therefore the potential energy will be maximum at the highest point of the steepest ascent.