Since the introduction of the ATP Masters series to the tour in 1990, it has become much easier to compare different seasons, and by every relevant statistic, what Novak Djokovic did in 2015 was the greatest singles season in the last 26-year era.
In 2015, Djokovic recorded an 82-6 record with 11 titles and reached the final of every event he played after dropping his quarterfinal match in the first tournament of the season.
His 82 wins are not a record nor are his just six losses. Twice in the last 26 years (Federer '06 & Sampras '93) has a player won more than 82 matches in a single season. Also twice in the last 26 years (Federer '05 & '06) has a player lost less than six matches, while maintaining a full schedule. Also his 11 titles are not a record. In 2006, Roger Federer notched 12.
However, that's not how success is determined in tennis. In tennis, when ranking the greatest seasons of all time, it only makes sense to use ranking points, since that's how the real rankings are defined. However, the problem is that the ranking system has changed several times since being instituted in 1973.
Thankfully, since 1990, the tour has remained similar enough that the conversion is fairly simple. There have always been four grand slams, nine masters series events, and a year-end final. Davis Cup, Grand Prix Cup, and smaller events add some complications, but those events do little to change the rankings of the top players in the end anyways, so best estimates are more than good enough.
Comparing the ranking points earned of all the year-end No. 1's for the last 26 years, Djokovic stands alone atop the following list.
1. 2015 Djokovic 16,585
2. 2006 Federer 16,295
3. 2011 Djokovic 13,630
3. 2007 Federer 13,630
5. 2013 Nadal 13,030
6. 2005 Federer 12,460
7. 2012 Djokovic 12,920
8. 2004 Federer 12,460
9. 2010 Nadal 12,450
10. 2008 Nadal 12,395
11. 1994 Sampras 11,950
12. 2014 Djokovic 11,360
The gap there between Djokovic's 2015 and Federer's 2006 is not big at all. Some might be tempted to say that Federer's extra title and better winning record more than make up for the ranking points gap and declare Federer's 2006 the greatest season ever. However, there is a better way to differentiate the two runaway best seasons ever.
College sports fans will be more than familiar with the term "strength of schedule." It is a measurement of the difficulty of a player's schedule based on the rankings of their opponent. For tennis, this is even easier than in college sports, because everyone is ranked and not just the top 25.
In a comparison of Federer's 2006 strength of schedule and Djokovic's in 2015, there is no question who had to face the tougher opponents. The average ranking of Djokovic's opponents was 35.8, while Federer's was 63.7. Almost 30 spots higher.
Of course, Federer did have a few opponents ranked high that drove up his average, but his easiest opponent was ranked 1078 and he needed a third-set tiebreaker to survive that test. Still, it is possible to use median opponent ranking as a better indicator in which Djokovic still holds a massive advantage with a median opponent ranking of 20 compared to Federer's 35.
Overall in 2015, Djokovic played 40.1% of his matches against players in the top 10, 52.3% against players in the top 20, and 59.1% against players in the top 30. Federer on the other hand, only matched up against top 10 peers 23.7% of the time, top 20 just 35.1% of the time, and top 30 only 46.4 percent of the time.
You don't choose your opponents in tennis, but both players played 17 events and Djokovic had the much tougher opponents and still managed to earn more ranking points than Federer.
In total, Djokovic won 31 matches against his peers in the top 10. Federer won only 18 in 2006. Djokovic's 31 wins against top 10 players smashes the record with the nearest amount in since 1990 being Rafael Nadal's 23 wins against top 10 players in 2013. The next highest was Djokovic's 21 in 2011.
With the highest degree of success and dominance the tour has ever seen against the toughest schedule in the ATP Masters Series era, little doubt remains that Djokovic's 2015 season was the greatest tennis season of the last 26 years if not in all of tennis history.