If the sole criteria for being included in this discussion is just peak level of play then this has to expand the discussion beyond just the normal GOAT contenders (who seem to be taking up the majority of the discussion). If overall achievements do not matter than I would say include the following players, especially if as hoodjem says, the criteria is peak play in one match.
Safin: he was an erratic mess of potential and his game had technical flaws that were exploitable (forehand that could be broken down, inconsistent at net)..but when he clicked he CLICKED. The 2005 AO is an example of that. His weaknesses in that tournament never really showed up against Fed, Fed played great..Satin was better on the day. Overall he was all over the place though.
Mary Pierce: yes I know this discussion is about men but I can't just not mention her. The 94 french SF against Graf fits so perfectly into what I interpret this discussion to be about. Pierce flat out destroyed the Graf forehand, neutralized her movement as much as could possibly be done, stopped her dead. Graf had nothing to beat her with. If Pierce could have played like that everyday she would have won double digit majors easily.
Ivanisevic: OK maybe he does not fit perfectly in but on grass with his serve he was a force to he reckoned with on his best day. It was either serve your best and pray for a miracle in the likely tie break or get beaten.
These are 3 examples that came to my mind when I read the thread title. Now..I am not saying their peak is better than Lavers, Borges,Macs etc. But if the thread is really about peak play for just 1 match...you can't really just center the discussion around just them. I have immense respect for all of them, but in terms of pure peak level for a single match..if cumulative stats are not to be considered..you have to look outside the box a little.
Just my opinion though.