Was this the strongest era ever in Men's Tennis with Big Four dominating?

It's very hard to answer what the strongest era in tennis was but it's clearly during a time when the big three of Novak Djokovic, Roger Federer and Rafael Nadal dominated.

For a good portion of that time, Andy Murray was in the mix as well with the four players big named the big four. They were winning the overwhelming majority of trophies between them and that's been the case for quite a while. 2012 was a year when each member of the big four won a trophy so that's a good candidate.

At that time all of them were pretty close to their peak though they each had their time when they were better. Djokovic only got better with time while players like Federer got worse with time. Murray was also pretty good at his peak with Nadal as well. All of them kind of lost a step or two with time.

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McEnroe sees Alcaraz as favourite for Roland Garros over Nadal and Djokovic: "Novak and Rafa are out of form"

Novak Djokovic was quite dominant back in 2016 having more points than the second and 3rd player combined. Some would say that was the best era of tennis due to the way he was able to rise on top with so many iconic tennis players. Below you can see the rankings from that time. In any case it's hard to say but it's clear that it was during the big three era. Which year is subjective.