I'll preface this by saying that, for now at least, Federer's accomplishments make him the best ever.
That said, it's intellectually dishonest to talk about spread of titles when the surfaces at W and the USO are highly correlative. If you don't believe me, look at the the winners/finalists of the two events over the last 25 years or so. There are a few exceptions, but there are a TON of similarities. There are also obvious similarities between the AO and the USO. The FO is the least correlative with any of the other majors.
People like to talk about how guys like Federer and Sampras have success on a diversity of surfaces, but they neglect to mention how they were having success on faster courts (it's no coincidence that each have 7 and 5 titles at W and USO, respectively).