There is something else I wanted to touch on though, and it's another area of grey in this "at his best" thing.
Generally, I look at player's best in average terms, not in one particular match (and that's something EL Dude alluded to earlier). For example, Tsonga's best ever performance was against Nadal at the AO in 2008. But the fact that he never got close to playing as well again actually indicates that he played ABOVE his level in that match. So is that his "best"? Yes, in the sense that he never played as well. But is it an indicator in the "at his best" conversation? No, because it's not a level he can reach for any sustained period of time. Not even a full tournament.
Every player has his day where they catch fire and everything they touch turns to gold. Kohlschreiber and Gasquet had a day like that against Roddick at the AO and Wimbledon 2008 respectively. I remember Cilic having a day like that against Nadal in some Asian hard court (or indoor) tournament in 2009. And hell, Robin Soderling got a couple of days like that, against Nadal and Federer at the 2009 FO and 2010 FO respectively (though in fairness to Soderling, he was able to sustain a high level for a couple of years, but nothing as ridiculous as his level in those matches).
That doesn't stop there. Even players who have had long periods of playing at a ridiculous level have had days where they were extra clicking. If we look at Federer, Nadal and Djokovic, they all have had specific patches where they played great even by their incredible standards (say, Federer in the second half of 2006, Djokovic's first half of 2011, Nadal's run in the 2008 starting with the clay court season till the Olympics, or his North American hard court last year, etc...). When I think of these players "at their best," that's what I think of: periods of sustained high level tennis.
And yet, even they had days where they exceeded even that. For example, Nadal's destruction of Verdasco in the Monte Carlo final in 2010 might just be the best display of clay court tennis I've ever seen. I honestly don't think Nadal's played a finer match in his career. Meanwhile, Federer's performances against Blake at the 2006 Masters Cup final and Roddick at the AO a few months later are ridiculous even by his standards. You could visibly see him having to stop himself from smiling after some of the shots he hit. Djokovic's match against Nadal in Rome 2011 was beyond anything he's ever played, and that really says something considering how well he was playing that entire year.
So, do you look at those matches as "Federer, Nadal or Djokovic at their best"? You could, since technically, they are. But I do think someone's "best level" has to be attainable. The level of tennis they played in those matches can only be attained a handful of times in a player's career. Which for my money, isn't enough to be a part of the conversation. Their performances in those matches were above their normal levels, even above their usual "best."