Simply put, there isn't one. By definition a coordinate system based on degrees is un-projected. In common parlance we say WGS84 is a "geographic" projection, but that's untrue, just for convenience.
I think I remember reading about a software or process for accurately working with elevation models in un-projected geographic space but I can't locate it right now. In any case it would have been an experimental or build it yourself from code kind of process.
Ahhh, found it: Development of a Global Slope Dataset for Estimation of Landslide Occurrence Resulting from Earthquakes (USGS). Page 4 describes the problem well
...the length of one degree varies depending on its latitudinal location. At the equator, a one-degree by one-degree block is reasonably square when converted to units of meters (111,321 meters in the x-direction by 110,567 meters in the y-direction ... but closer to the poles the distances in the x-direction grow smaller as a function of the cosine of latitude, owing to convergence of the meridians. Most GIS packages, ArcGIS included, operate only on square pixels, and so using a factor to adjust the x, y, or z dimensions to a common unit is not possible.
The paper goes on to describe the specific calculations and software tools (gdal, python, numpy) they used to workaround this fundamental issue. The paper doesn't include the code, but if asked nicely they might share. In any case though I'd probably just ask where the results are, being the USGS it's probably already online somewhere. :)