When this question comes up my immediate reaction is normally Serena Williams because she absolutely bossed her sport for so long. And tennis is pretty global, which I also think is a factor. She's still won more major championships than Djokovic (I think) once you factor in she plays doubles and mixed doubles too, which again is a positive factor in a question like this.

But I'm starting to think Djokovic is edging it now. He has had to win most of those medals when Federer was around, and I think he's overtaken Serena now on singles titles.