Really hard for me to decide on a professional tennis GOAT. Seems like there was a Golden Age around 40-50 years ago that included Boris Becker, Bjorn Borg, McEnroe and Connors, Stefan Edberg and Ivan Lendl, others who were very popular but aren't immediately coming to mind. Most of them had stretches where they seemed unbeatable
Then we moved on to Agassi and Sampras and Chang, and Sampras seemed like he might be the GOAT, unless we go back to Rod Laver and some of his contemporaries.
And yes, Federer racked up a lot of impressive wins, and I'm glad he had Nadal as a foil, and later Djokovic.
I suppose whoever has the most wins and majors gets to claim the GOAT title, so if that's Fed, that's okay by me.
Fed is two behind Nadal in majors and one behind Djokovic now. A lot of his majors came at a time when he had no real challengers, not that that is his fault, he can only beat the person in front of him but it does hurt his legacy when comparing his career to the other two. Either way, definitely a legend of the game.