WebMapServiceImageryProvider EPSG code

I have GeoServer that provide raster data for Cesium. Currently my data is in EPSG:3857 projection.
If Cesium would like to use the data in EPSG:4326 by default there is the transformation overhead on the client side, I would like to side step this.

What is the best projection for the data to be served in from the server?

Regards