Type of Plasterboard to use in a bathroom?
Standard drywall is just fine. In fact, as a builder we rarely used moisture-resistant drywall even above shower surrounds. Proper ventilation and good quality paints do the job just fine. We never had callbacks. 

In my opinion, if you think you need moisture-resistant drywall, such as for tile backing, you should be using cementboard. Drywall of any type isn't appropriate for areas that regularly get splashed.