when working with OGC WMS services (such as those provided by GeoServer, but also available in ArcGIS for Server), it is preferred to use a basemap that is in EPSG:4326 projection. this is because that is the only required projection to be supported by WMS services. Anything else (such as EPSG:102100) and you may run into WMS services that cannot reproject (which is done server-side).