Laver played in a time when three of the four Grand Slams were played on grass.

Next, was Borg, who played in a time where you rarely played the Australian Open unless you had won the other three titles.

Sampras played in a time where tennis was contested in many different ways, and he prevailed.

Then, Federer who, in his prime, has played with mainly one other winner, Rafael Nadal.

Each of these players times have advantages and disadvatages....

Laver was best on grass—a huge advantage. But during six years of his prime, he wasn't allowed to play and some don't count those matches as wins.

Borg didn't play the Australian Open, limiting the amount of titles he had. He also had a whole lot of competition, with Connorrs, McEnroe, and Lendl. But, Borg had some advantages, at the very start of his career, he had almost no competition, and won a few Grand Slams then.

Sampras played with the first batch of new equipment, giving him a slight advantage. But, tennis at his time had a lot of people who were playing very well.

Federer's disadvantage is also his advantage. Having one rival is not very good—if he consistently beats you, that's even worse—But, since not too many other people are contesting, he only has one challenge.

Well, these four players are probably the best four ever, but the order is completely unknown. You can't compare the ancient Roman army, that many consider the greatest ever, to the present-day American army, can you? Well, it's the same with tennis. You just can't compare eras because things change a lot from era to era.