"The best" is always a subjective term—especially when applied in men's tennis. From training regimes to court surfaces, from tennis rackets to nutritional considerations, things change from one era to the next.
As a case in point, look at the number of points awarded for tournament wins this season compared to last season. Furthermore, due consideration needs to be given to the lens through which the comparison is being made. Here are a few of the conventional candidates for the title of "the best."
If you use the number of Grand Slam championships as a measure, Sampras tops the list with 14. He also holds the record for most wins at a single Grand Slam event; he's won seven Wimbledon titles.
Borg doesn't get nearly enough "air time" in these types of discussions. He has the best career match winning percentage at 83 percent. Furthermore, Borg only ever played the Australian open once, in 1974. Considering his career ended in 1981, with 11 Grand Slam titles, who knows whether he may have won more Grand Slams and set the bar too high for even Sampras to overcome.
The only male player to have a career Golden Slam (an Olympic gold medal and all Grand Slam singles titles), Agassi is also the only male player to have won all four Grand Slam singles tournaments on three different surfaces.
Federer holds a slew of records across multiple categories including having appeared in 10 consecutive Grand Slam men's singles finals. He's won 13 Grand Slam singles titles to boot.
The only male player to have held Grand Slam titles on three different surfaces simultaneously, Nadal belongs in this discussion despite his relative youth.
These are only Open era records; do we discount off-hand the accomplishments of the players who came before?
In the quest to define the greatest player, do accumulated titles count for more (Sampras, Federer), or should we prioritize the ability to adapt and modify your game to different surfaces (Nadal)?
How about climbing to the top of the rankings, only to drop out of contention and then reclaim the top spot (Agassi: 1995, 1996 world No. 1; 1997 world No. 141; 1999 world No. 1).
My personal view is that in any given era—and "era" is always defined in hindsight, everyone thought that it was going to be the Roddick-Federer era and look how one-sided that turned out—one player, perhaps two, stands out as being the best for that period. It's speculative at best and churlish at worst to try and define an all-time greatest ever.
Facts and figures may be there for the taking, but the current best will always be usurped in one way or another. As an observer and spectator of the grace, grit and sometimes grunt of the modern game, the best thing to do is sit back and savour the tantalizing tennis that gets dished up.