What do you use for exterior drywall?

The terms Exterior and drywall are not compatible. Drywall is used on the inside of the wall and is protected by a suitable weatherproof surface on the exterior of the wall framing such as lapboards, stucco, exterior sheathing etc. All of these have a moisture barrier behind them to prevent water/moist air from contacting the drywall. Drywall will disintigrate and allow mold/mildew and fungus to grow when moist. Eventually it will fail completely. So-called "green board" drywall is used in bathrooms and areas subjected to moisture but only outside the bath or shower area. It is not suitable for anything other than a backing material if used in the wet area of a bathroom. It can reside behind cement board or other materials with waterproof qualities. Never adhere tiles directly to green board unless it is outside the wet area of the bath/shower.