It all depends on what your definition or criteria for "Greatest" is.
If we're just talking the freakiest, most massive, with the most Olympias, then RC would probably win.
If considering bringing the popularity of bodybuilding more mainstream, accomplishments aside from bodybuilding, then Arnold wins hands down.
Or, if we are looking for the more aesthetically pleasing we could choose from a number of past bodybuilders. It all depends on what type of physique you most admire. Beauty is in the eye of the beholder.