It's impossible to compare different eras. Jack had Arnold Palmer, Gary Player, Lee Trevino, Tony Jacklin, Ray Floyd, as rivals, and later Johnny Miller and Tom Watson and Ben Crenshaw came along. And towards the end that guy from Spain, Seve Ballesteros. Most of those were multiple major winners. Koepka, Speith, McIlroy, are young guns with multiple major wins... Other than them, Mickelson (five majors) is now 48 and Ernie Els (four majors) 49. It will be an even bigger deal than today if Phil manages a another major - I would love to see it but am not very confident. Els barely even plays a full schedule any more.
But to me the biggest factor in the decision as to whom is the Greatest of All Time, it is Nicklaus' 19 second place finishes in the majors in addition to his 18 wins. That means he was virtually always in contention in 37 of them.
No argument Jack Nicklaus was the best player of his era. The game has changed a lot now. The courses are longer, greens are much faster, equipment is better by orders of magnitude over the wooden clubs of Nicklaus era.
As I said before, during his 20’s and early 30’s Tiger Woods won by larger # of strokes his majors than anybody has done in a century. If he had not messed up his life, who can tell how many majors he would have won. As one who has played golf for 60 years, and for 20 years full time 5 days a week, and knowing something about physics with my master’s degree in mechanical engineering, in my mind, Tiger Woods is the best golf I have seen.