The pre-open wood racquet era has nothing whatsoever in common with the accomplishments of the open-era wood racquet days. This is because the number of people competing for the major titles seriously went from dozens to millions. Using Slam totals to measure anyone from this era is silly, since nobody from that era really cared about building up a resume that way. Most good players skipped the Australian entirely.
Those days, in turn, have nothing whatsoever to do with the post-wooden (i.e., "graphite") racquet, pre-2002 era. This is because the additional power enabled by technology allowed a certain skillset (power serving and S&V) to absolutely dominate the faster courts, while a completely different skillset (baseline consistency) completely dominated slow courts. Since two of four majors were typically pretty fast (Wimbledon and the U.S. Open), one was slow (the French), and one couldn't decide what it wanted to be (Aussie), fast court specialists had an advantage if they could stay near the top of the game, as Pete did. It was also an era when nobody much cared about the total number of career Slams anyone had (until Pete). Slow court specialists routinely skipped or tanked Wimbledon entirely, and fast courters often did the same at the French. Yannick Noah skipped Wimbledon almost every year because he didn't like grass or England.
And those days, then, have nothing whatsoever to do with the post-2002 era of homogenized surfaces and technologically-mandatory power baselining. With all four slams playing by and large the same, it's an era when counting Slams against EACH OTHER makes perfect sense. When one style of play works best at every one of the game's biggest tournaments, the guy who wins them has an obvious and unassailable claim as top banana within the era. But those same factors mean that being slightly better than the rest of the field all but guarantees dominance in terms of Slam trophies in a way it never could before 2002. Thus, it makes no sense to compare Federer's slam totals to Sampras's, while it does make a kind of sense to compare them to Djokovic's and Nadal's.
I'll let the historians debate who the GOAT of the pre-open era is. Pancho Gonzalez has a claim, I guess.
Laver and Borg probably have the strongest claims to GOAT in the open, wood-racquet era.
Sampras probably has the best claim to GOAT of the graphite, pre-2002 era.
And Federer has it for post-2002, though he's got a couple guys in pursuit.
Comparing those various greatests of their times (GOTT's, I guess) to one another, or any other players from cross-era, is just wankery. The levels of competition, and kinds of competition, and things any one player could possibly hope to achieve vary too greatly from era to era.
But that's why I'd probably call those three or four guys the top of any sane GOAT list that tries to cross boundaries. Djokovic's extra slam vs Borg is less than meaningless. He's not chasing Borg, who for all intents and purposes played an entirely different sport, with different goals. He's chasing Federer and Nadal only. And only the guy who comes out on top can reasonably be considered a peer to Sampras, Laver, and Borg.
