Orthometric heights related to the geoid, are important in many engineering projects. The use of Global Positioning Systems (GPS) does not give the heights of the points on the surface of the geoid but gives them on the surface of the reference ellipsoid. Geoid undulations should be offered to obtain the needed orthometric heights from their corresponding available ellipsoidal heights. The aim of this research is to obtain the best among the Global Geopotential Models (GGMs) that give the results closest to the observed geoid heights over two study areas. Firstly, in Egypt and secondly in the Kingdom of Saudi Arabia (KSA). In the first study area, RMS of the differences between the observed undulations (GPS/levelling) and those from GOCO05c, XGM2016, and GECO are the lowest values among the ten tested GGMs. In the second study area, RMS of the differences between the undulations from KSA-GEOID21 and those from XGM2019e_2159, XGM2016, and XGM2019 are the lowest among the ten tested GGMs.