@Federberg, I hear you about cross-generation comparisons being very difficult, but that doesn't mean it is impossible or pointless to talk about (or at least no more than any such conversations are pointless!).
I still think the key is in considering
relative dominance. We can't compare the stats of, say, Bill Tilden to Rafael Nadal, but we can compare how dominant they were relative to the context they played in. Probably the best way to do that is via rankings. While the weight of different tournaments changes, the #1 ranked player is still always the best player in the game (at least usually!). We only have computerized rankings from 1973 to the present, but there were still consensus #1 players going back and some sites (e.g. Tennis Base) have rankings going back to 1877.
I'm going to use Tennis Base's rankings, which I assembled into a chart for every player that was #1 at least once. Let's position a point system like so:
#11-20 ranking: 1 point
#6-10 ranking: 3 points
#3-5 ranking: 5 points
#2 ranking: 10 points
#1 ranking: 20 points
The point system is arbitrary, but I think is decent for weighing dominance. According to this system, here are the twenty-one greatest players of all time (couldn't exclude Andre):
1. 301 Bill Tilden
2. 202 Rod Laver
3. 185 Pancho Gonzales
3t. 185 Ken Rosewall
5. 165 Roger Federer
6. 164 Tony Wilding
7. 150 Josiah Ritchie
8. 137 Pete Sampras
9t. 131 Laurence Doherty
9t. 131 Jimmy Connors
11. 128 Rafael Nadal
12t. 126 Ivan Lendl
12t. 126 Novak Djokovic
14. 117 John McEnroe
15. 116 Don Budge
16. 111 Pancho Segura
17t. 101 Bobby Riggs
17t. 101 Bjorn Borg
19. 100 Harold Mahony
20. 97 Harry Barlow
21. 94 Andre Agassi
Again, no such system is without serious flaws. I see this as just another angle to consider. But a few things emerge:
1) Look at how far Tilden is ahead of the field. According to this system, he really is the Babe Ruth of tennis. He was #1
thirteen times. Next most was Laver with eight, Pancho Gonzales and Tony Wilding with seven each, then Doherty, Sampras, and Federer with five each (TB has Rios as #1 in 1998).
2) Josiah Ritchie? Perfect example of a great player who is forgotten because he didn't win any Slams. But he was a terrific player who won tons of tournaments.
3) Interesting how Lendl and Novak are tied right now. Of course Novak isn't done yet and will pass Ivan this year.
4) If Rafa finishes #1 or #2 this year, he passes Sampras.
5) And I know Ricardo is going to throw a fit because this system has Riggs and Borg tied. Well, according to TB rankings, Borg was #1 three times, #2 three times, #3-5 two times, and #11-20 once. Riggs was #1 three times, #2 twice, #3-5 three times, #6-10 once, and #11-20 three times. So they both were the best player three years, with Borg having a slightly more top-heavy distribution after.