Waldner 2018
Many, perhaps most, consider Jan-Ove Waldner the greatest player of all time. I've never played him, but I've met and talked to him many times, even had lunch with him and other top Swedes when I interviewed them for an article. What would happen if he were in his prime today?
There's no way of making a perfect comparison. For one thing, Waldner played nearly his entire career with a 38mm ball, while we now use 40mm. Equipment has also gotten better. (Hidden serves are now "illegal," but it isn't really enforced, so that part won't affect him. Games to 11 instead of 21 won't make much of a difference.) So how would he do? Keep in mind that nearly always the next generation is "better" than the previous generation, with better techniques, better training, and better equipment. Only a true phenom like Waldner could hope to compete with players a couple generations later. (Of course, if Waldner were to have developed in modern times, he'd have access to these better techniques, training, and equipment, but we're going to look at him as he actually was, not as he might have been.)
First, note that "greatest" is not the same as "best." Victor Barna is one of the greatest players of all time - he won Men's Singles at the Worlds five times. But I'm pretty sure that if I could go back in time when I was at my peak and play him when he was at his peak, using the rules at the time, I'd beat him easily, since I'll using Tenergy and throwing loops and serves at him that he's never seen before, and he'll be stuck with a hardbat. Similarly, the best modern swimmers are all faster than Mark Spitz, but he won seven gold medals at one Olympics, setting a world record in each of them. The best modern swimmers are better than Spitz, but Spitz was greater than all of the ones not named Phelps or Ledecky.