The Peak ELO rating by Ultimate Tennis Statistics is NOT an objective way to measure the BOAT/highest peak ever. Why? Because the ELO rating formula considers all the ATP points achieved by a particular player, including Masters 1000 and Masters 500. The problem is that it underestimates the value of Grand Slams.
It is mathematically possible for a player with only 2 Grand Slams in a year to have a higher peak ELO than a player with the Calendar Year Grand Slam. In effect, if the player who wins only 2 Grand Slam in a calendar year wins many Masters 1000 and Masters 500, he can have a higher peak ELO than a player with the Career Grand Slam but very few Masters 1000/Masters 500.
http://www.ultimatetennisstatistics.com/peakEloRatings
McEnroe has a higher peak ELO than Laver, Federer and Nadal, despite the fact that he never won the CYGS or 3 Grand Slams in a calendar year like Federer or Nadal. So no, peak ELO is not a synonym with higher peak level, since the ELO rating underestimates the value of Grand Slams. A man with only 2 Grand Slams in a year (McEnroe) doesn't have a higher peak level than Federer or Nadal, who won 3 Grand Slams in a calendar year, let alone Laver.
So if peak ELO is NOT a criterion of BOATNESS... then what does determine who is the BOAT?
In order of relevance, these are the only two criteria of BOATNESS:
1. H2H in Grand Slams against your main career rivals. It measures the highest peak level against the greatest rivals. Grand Slams tournaments are universally considered to be more relevant than other tournaments. Analogously, H2H in Grand Slams shoudl be considered more relevant than H2H in other tournaments. Otherwise, we would be displaying a double standard logic. Grand Slam achievements should always be considered more relevant than achievements outside Grand Slams.
2. Number of consecutive Grand Slams. Another indicative of peak level, but not so strong, as it is possible to win many Majors without facing top rivals,
CONCLUSION
Nadal is the BOAT: Nadal leads the H2H in Grand Slams over Federer and Djokovic. Nadal leads the H2H in Grand Slams 9-3 over Federer (including 4-3 outside clay) and 9-5 over Djokovic (including 2-1 at the US Open). That is, Nadal has won 75% of his Grand Slam matches against Federer, and 64% of his Grand Slam matches against Djokovic. That's brutal domination.
Rod Laver is the second in the BOAT list: Only player to win 4 Grand Slams in a row twice (not only once like Djokovic). Even if Djokovic achieved it on 3 surfaces, it does not compensate Laver's extra CYGS. Even Nadal won 3 Grand Slams on 3 surfaces in 2010, and it does not mean Nadal's 2010 is better than Laver's CYGS. In sum, 2 CYGS >>>>1 NCYGS.
Djokovic is the third in the BOAT list: Djokovic has no argument to be the BOAT, since he has a losing H2H in Grand Slams against his greater rival (Nadal). No BOAT can have a losing H2H in Grand Slams against his greatest rival. The "only on clay" excuse is ridiculous. Clay is a tennis surface, and Nadal also leads 2-1 at the US Open. And even Laver has a stronger BOAT argument, since 2 CYGS > 1 NCYGS.