Unfortunately for those that are incredibly good on clay, almost half of the ATGs aren't amazing on the surface. Even less-peak players on the surface like Agassi will have quite good records against those 5. Someone like Djokovic might go about even with Federer, Agassi, Lendl, Wilander and say 2-8 against Nadal and Borg combined, but will go something like 8-2 against the other 5. 64-46 for Djokovic-level players (conservative estimate) but something like 84-26 for Borg (something insane like 100-10 for Nadal). It's still a commanding lead, but less than it could've been had all of the ATGs been Agassi-level on clay, as they would've provided more competition for Nadal's rivals - he would've smashed them either way.
Hard Court: Unfortunately for the best here, almost everyone is a contender and HC is a very neutral surface, not to mention that the ATGs have 6 peak HC players. Assuming they go about even, dominate the non-contenders 10-0, and beat the non-best contenders 8-2, they still have a record of about 75-35. That's probably a liberal estimate, so the real figure is probably slightly lower. Of course, some will do better and some will do worse, but all 6 will hover around that mark somewhere. Not a significant gain for the best here, but a terrible blow for the worst (Borg - 21-89 at best)
Grass: Similar situation as HC, but not as bad. Federer/Sampras/Borg make some headway, but the other contenders are still stiff competition, unlikely to be blown away worse than 7-3 or at very worst 8-2. No 9-1 or 10-0 blowouts other than for Lendl and Wilander.
The best would at least need to be a contender on each surface OR peak on 2/3, so that eliminates Lendl, Connors, Wilander, McEnroe, Becker, Edberg. Remaining: Nadal, Djokovic, Federer, Sampras, Borg, Agassi. Of these, Borg probably loses a lot of ground on HC. It's possible he makes it up on clay + grass, but I think in the end it's probably not enough. Agassi doesn't hit best of the best in any category, so he's trying to break 0.500 by not losing to the best on each surface as much, which I think is a losing strategy.
In the end, you get Nadal, Djokovic, Federer, Sampras. Compare Sampras to Federer - roughly equal on grass and hard courts, but Federer holds his own on clay much better. Sampras is out. Big 3 left.
Nadal gets something like 100-10 on clay, about 49-61 on HC, and 45-65ish on grass. About 194-136.
Djokovic goes about 64-46, 80-30, 59-51 for maybe 203-127.
Federer goes roughly 60-50, 80-30, 76-24 for 216-114.
These figures are complete estimates, and I might've been a little lenient for Federer's clay numbers and HC numbers - but even if you take away 5 wins on clay and 10 on HC, he still has 201-129, ahead of anyone else I think (given Djokovic would also lose some HC matches if I was easier on Federer).
Overall, ATGs just struggled on clay more so a middle-of-the-pack ATG there gets a lot of "undeserved" wins and Nadal's obscene peak on the surface counts for a bit less. Exactly the opposite is the case for HC, as ATGs excelled so much there that someone that's middle-of-the-pack actually ends up with a significant losing record simply because there were so many peak players. Double-edged sword, of course, as those players also need to player each other.
TLDR: The best candidate for this sort of competition is an ATG with a GOAT HC level, ATG grass level, and at least a middling clay level. The best suited candidate for this is Federer, followed by Djokovic. Nadal comes close simply by brute force - he's so monstrously good on clay that it makes up for the relative weakness of his grass/HC game. Only problem is there's so many good grass/HC players that it affects his overall score.
