Most quadcopters don't allow for the motors to be reversed - it's very rare for the motor controllers to support this unless the quadcopter is specifically designed for inverted / "3D" flight with custom motor controller firmware and bidirectional / 3D props.

"What motor inputs would cause the hardest recovery situation" is a pretty fun question, though - I think the best you could do would be to throttle the two motors rotating in a given direction to zero (both CW or both CCW), which would induce instability in multiple axes at once. But there's probably something more creative I'm not thinking about!