Slash is better than Keith Richards in terms of guitar skill, although this isn't an easy conclusion to come to. Slash has a technical skill that outpaces Richards' by a long shot. Keith Richards definitely wins in terms of legendary people (no offense Slash), or people you'd like to hang out with (again, sorry), but not in skill.
It is unfortunate that celebrities are put on such a pedestal that we often discuss them as better or worse than each other and find them better than non-celebrities. Neither Slash nor Keith Richards are better than one another or anyone else, they are merely different people. While people may like the work produced by one over the other, that does not make the individual better than the other.