i have already stated what i think is the most pain in a physiological sense, but i think the worst sensation to be condemned to would be the destruction of the semicircular canals in your ears.
they provide the x plane and the y plane and the z plane stability that tells your mind where you are, and that you are still.
when you spin around on the spot (with your axis being upright(or vertical (rotating with respect to the x axis))) , you disrupt the fluid circulating through your x axis canal, and it eventually starts to spin with you due to surface tension. when you stop spinning, the fluid continues to spin and loses it's momentum after a time, but it makes you feel like you are spinning in the opposite direction for a while. the semi circular canal on the vertical axis is disrupted.
if you were doing cartwheels for a few hundred meters, then when you stopped, you would fell like the world is spinning like a dial in front of your face. the axis of rotation is parallel to the horizon, but the perceived effect is that the world is spinning like the face of a clock (in time lapse).
if you decided to somersault for a mile or so, then when you stopped, you would feel like you are still tumbling end over end (from behind to "in front") for a while. this is a z axial disturbance due to the same fluid dynamic that caused the other axial aberrations in my earlier examples of spinning.
when the semi circular canal is destroyed, there is a sudden loss of stability in all 3 axes of orientation, and the sufferer is condemned to a world of disorientation and almost fatal nausea for the rest of their lives..