Which factor plays the greatest role in determining the climate in a highland region?
Probably elevation and the impact a highland area has on rainfall patterns.
As you go higher in elevation, the atmosphere gets colder at a regular rate called the environmental lapse rate. So, any highland or mountainous region tends to get colder as you go higher up.
Highland and mountain regions also impact rainfall patterns. Winds that flow towards an upland area get lifted up the windward side and if there is moisture in the air, it will tend to fall out as rain as the air moves up the mountain or highland side. Similarly, as the air moves down the side of a mountain or upland region, as it descends the air is compressed, it dries out and tends to produce dry climates on the lee side.