Federer is way ahead of everyone (as of right now), but I think you can make a strong case for 4 players as #2 : Borg, Sampras, Nadal, and Djokovic. Here's the pro con for each of them.
Borg
Disclaimer: He "only" won 11 slams, but the top pros didn't really play Australia including Borg, so he won 11 slams with 3 slam events a year. This would equate to winning 14.67 slams with 4 slam events. Or, as some people say, the YEC was really the "4th slam" so he won 13 slam equivalents. Thus, I will consider him being on par with Sampras and Nadal slam count wise.
Pros:
- Had sustained dominance for a long time at least for 1978-1980. Overall won 11/27 slams he played in.
- Won the channel slam 3 years in a ROW. Won 3 slams without dropping a set, 1976 wimbeldon, and 78,80 RG.
- 5 straight wimbeldons, only ever lost to one person at the French open. Arguable clay goat, and only behind Federer and Sampras on grass.
- Massive impact on the sport, was a cultural icon /star on a level Djokovic and Sampras never came close to. Only Federer may have been a bigger figure in the sport.
- Extremely tough mentally, won something like 13 straight fifth sets.
- Huge what-if: I don't fully buy this line of thinking, but some say his greatness is magnified by retiring at 25. The ATP really was terrible in not letting him come back with a more limited schedule after his burnout. Could likely have tacked on some French opens at least. Don't really see who beats him in 82-83.
- Prodigal teenager, only Becker may have accomplished more as a teenager.
Cons:
- Never won the U.S open. One of the biggest events in tennis, and Borg could not win it. He did get unlucky to be injured before the 78 final, but this isn't a game of what-ifs.
- Left a lot on the table, so his accomplishments could have been a lot greater.
- Not enough time at #1. The rankings were terrible in the 70s, so he was really #1 for more than 101 weeks, but certainly not #1 as long as Sampras, or even most likely Djokovic.
Sampras
Pros:
- Co-GOAT at wimbeldon and USO with 7 and 5 titles. 2 of the 3 biggest events in tennis.
- 6-straight YE #1 (most total #1 YE), and 286 weeks, which is far ahead of everyone else on this list.
- Dominated every rival, from Agassi to Becker to courier to Chang to ivanisivec. Turned Agassi into a meth addict. only rafter-Sampras wasn't a one-sided rivalry.
- Phenomenal competitor : won 14/18 slam finals, and 12/13 in his prime. Only peak Agassi beat him at 95 USO. Threw up on the court in a fifth set tiebreaker and still won the match.
- Storybook career ending going out on top.
Cons:
- Was a total non-factor on clay
- never had a season on the level of Djokovic 2011/2015, or Nadal 2010, or Borg 78-80 IMO.
- More inconsistent than Djokovic or Borg, especially Djokovic.
- Losing H2H with Federer
Djokovic
Pros
- Nole slam: single greatest exhibition of dominance in tennis since Laver's grand slam. Very remarkable feat.
- 2011/2015 were both all time great seasons that were above any Nadal or Djokovic season, and likely Borg too.
- Australian open GOAT, 2nd best harcourt player ever.
- incredible consistency in his prime, 2nd only to Federer in this aspect and far ahead of Sampras and Nadal.
- DOMINATED close to PEAK Nadal, and had a positive H2H with Federer (don't put much weight on this).
Cons:
- Less slams than the other 3.
- Lost FOUR slam finals to non all time greats Murray and Wawrinka in his PRIME. Sampras and Borg had no such losses (maybe 2000 USO for Sampras), and Nadal only 1 such loss (Wawrinka 2014).
- way Less impact on the sport than Borg and Nadal, and likely Sampras too. Don't count this too much, but others may.
Nadal
Pros:
- Dominated clay for a decade, in a way no one has dominated any surface. Won Roland Garros twice without losing a set, and did the channel slam in those 2 years as well.
- Career slam like Nole, and career golden slam, which only Agassi did. Also accomplished career slam at a younger age than anyone by a good 3 years.
- Had a great H2H against Federer (don't put much weight on this), and beat prime Federer in two epic battles off of clay (I do value this). Also beat Djokovic in most big matches (7-3 or 8-3 in matches where winner won the slam).
- Also a prodigal teenager.
- Injury questions: maybe he could have won more slams if he was able to stay healthy. I personally think this is bad logic, because durability is a trait that is important in tennis.
Cons:
- Least time at #1 of everyone.
- Did the least off his favored slam/surface compare to all the others.
- Was only consistent for a few years, rarely totally dominated the tour like the others did.
Who do you guys have as no. 2? What about 3-5? I did this list in my order (Borg, then Sampras, then Djokovic, then Nadal), and think djokovic will jump ahead of Borg and Sampras with 1 or 2 slams, and maybe a run at no 1. Nadal for me would need to spend some time at no 1 and win slams off of clay. I think any ordering of these four players is reasonable however.
Borg
Disclaimer: He "only" won 11 slams, but the top pros didn't really play Australia including Borg, so he won 11 slams with 3 slam events a year. This would equate to winning 14.67 slams with 4 slam events. Or, as some people say, the YEC was really the "4th slam" so he won 13 slam equivalents. Thus, I will consider him being on par with Sampras and Nadal slam count wise.
Pros:
- Had sustained dominance for a long time at least for 1978-1980. Overall won 11/27 slams he played in.
- Won the channel slam 3 years in a ROW. Won 3 slams without dropping a set, 1976 wimbeldon, and 78,80 RG.
- 5 straight wimbeldons, only ever lost to one person at the French open. Arguable clay goat, and only behind Federer and Sampras on grass.
- Massive impact on the sport, was a cultural icon /star on a level Djokovic and Sampras never came close to. Only Federer may have been a bigger figure in the sport.
- Extremely tough mentally, won something like 13 straight fifth sets.
- Huge what-if: I don't fully buy this line of thinking, but some say his greatness is magnified by retiring at 25. The ATP really was terrible in not letting him come back with a more limited schedule after his burnout. Could likely have tacked on some French opens at least. Don't really see who beats him in 82-83.
- Prodigal teenager, only Becker may have accomplished more as a teenager.
Cons:
- Never won the U.S open. One of the biggest events in tennis, and Borg could not win it. He did get unlucky to be injured before the 78 final, but this isn't a game of what-ifs.
- Left a lot on the table, so his accomplishments could have been a lot greater.
- Not enough time at #1. The rankings were terrible in the 70s, so he was really #1 for more than 101 weeks, but certainly not #1 as long as Sampras, or even most likely Djokovic.
Sampras
Pros:
- Co-GOAT at wimbeldon and USO with 7 and 5 titles. 2 of the 3 biggest events in tennis.
- 6-straight YE #1 (most total #1 YE), and 286 weeks, which is far ahead of everyone else on this list.
- Dominated every rival, from Agassi to Becker to courier to Chang to ivanisivec. Turned Agassi into a meth addict. only rafter-Sampras wasn't a one-sided rivalry.
- Phenomenal competitor : won 14/18 slam finals, and 12/13 in his prime. Only peak Agassi beat him at 95 USO. Threw up on the court in a fifth set tiebreaker and still won the match.
- Storybook career ending going out on top.
Cons:
- Was a total non-factor on clay
- never had a season on the level of Djokovic 2011/2015, or Nadal 2010, or Borg 78-80 IMO.
- More inconsistent than Djokovic or Borg, especially Djokovic.
- Losing H2H with Federer
Djokovic
Pros
- Nole slam: single greatest exhibition of dominance in tennis since Laver's grand slam. Very remarkable feat.
- 2011/2015 were both all time great seasons that were above any Nadal or Djokovic season, and likely Borg too.
- Australian open GOAT, 2nd best harcourt player ever.
- incredible consistency in his prime, 2nd only to Federer in this aspect and far ahead of Sampras and Nadal.
- DOMINATED close to PEAK Nadal, and had a positive H2H with Federer (don't put much weight on this).
Cons:
- Less slams than the other 3.
- Lost FOUR slam finals to non all time greats Murray and Wawrinka in his PRIME. Sampras and Borg had no such losses (maybe 2000 USO for Sampras), and Nadal only 1 such loss (Wawrinka 2014).
- way Less impact on the sport than Borg and Nadal, and likely Sampras too. Don't count this too much, but others may.
Nadal
Pros:
- Dominated clay for a decade, in a way no one has dominated any surface. Won Roland Garros twice without losing a set, and did the channel slam in those 2 years as well.
- Career slam like Nole, and career golden slam, which only Agassi did. Also accomplished career slam at a younger age than anyone by a good 3 years.
- Had a great H2H against Federer (don't put much weight on this), and beat prime Federer in two epic battles off of clay (I do value this). Also beat Djokovic in most big matches (7-3 or 8-3 in matches where winner won the slam).
- Also a prodigal teenager.
- Injury questions: maybe he could have won more slams if he was able to stay healthy. I personally think this is bad logic, because durability is a trait that is important in tennis.
Cons:
- Least time at #1 of everyone.
- Did the least off his favored slam/surface compare to all the others.
- Was only consistent for a few years, rarely totally dominated the tour like the others did.
Who do you guys have as no. 2? What about 3-5? I did this list in my order (Borg, then Sampras, then Djokovic, then Nadal), and think djokovic will jump ahead of Borg and Sampras with 1 or 2 slams, and maybe a run at no 1. Nadal for me would need to spend some time at no 1 and win slams off of clay. I think any ordering of these four players is reasonable however.