When the roller coaster cart is at the top of the roller coaster, which best describes its energy.

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.