Well, on one hand, it's hard to dispute a perfect record! But I know nothing about Greco's opponents -- were they the best players of his time? Or just some guys down at the coffeehouse?
I think in the more modern era, Steinitz is the pretty clear choice. He invented modern strategy, and just ran circles around his contemporaries -- who were pretty good players in their own right.
Okay, this choice is easier than if I had asked who is the greatest chess player of all time. Any way I just wnat your opinion to whether I'm wrong or not about my choice of Gioachino Greco who went +78 -0 =0 which is a perfect record, he lived in the 17th century.