The ramp with the smallest mechanical advantage is the steepest.
In fact, the ideal mechanical advantage of a ramp is given by the ratio between the length L of the ramp and its height h:
[tex]IMA = \frac{L}{h} [/tex]
The steepest ramp is the ramp with largest height h, therefore it is the ramp with the smallest mechanical advantage, because its ratio L/h is the smallest.