When the roller coaster is at the very peak of it upward motion, it stops for a very short moment. It can no longer go any further upwards.
That being so, there (in theory) can be no kinetic energy. But there is energy: the coaster does not stay stopped. It has full potential energy described by mgh. M and g are constants. The height (h) is at a maximum. The upward gravitational kinetic energy is at a minimum -- 0.
