I'll go with Bjorn Borg, due to blazing speed, incredible quickness, great stamina, anticipation, and sublime footwork. He had an ability to finish shots from wild positions even with the old paradigm of wood frames from the baseline against great net play. He had great passing shots largely due to his court coverage. I'd also put Nadal at perhaps #2 with Djokovic and Federer a bit behind Nadal. Federer's footwork gives him a big advantage over Djokovic especially on grass courts, so that's to his credit. I consider court-coverage to be a broad topic, with top speed as only one thing to consider. Yet, even on that score, Borg was a bit faster than Nadal, with better quickness and stamina as well. This is with those old, cool looking Diadoras too! Borg would have thrived in the conditions of today, with a slow hard court major, and frames that allowed for a shift in favor of players that adopted a style of play that players such as Borg and Lendl brought to prominence.
Borg vs. Vilas (thanks Krosero)
(Borg goes offscreen)
Borg vs. Lendl
(This was indoors on fast carpet at MSG in the 81 Masters YEC final. That was the 4th biggest tourney of the year and there were with 18K in attendance.)
(81 FO final)
Borg vs. Connors