Marin Cilic at the 2014 US Open was unstoppable. In the second week he straight-setted Berdych, Federer, and Nishikori in the QF, SF, and F. He was about as dominant as I've seen over a few matches. But I bring him up not to be contrarian but to point out that any reasonably good player can play light's out for a few matches. Cilic earned his Slam trophy, but is really a second tier player who played really, really well for a week.
If cali were around he'd probably point to David Nalbandian at the end of 2007. Isn't he the only player in tennis history to beat Rafa, Novak, and Roger all in the same tournament?
But if we take the idea that tennis evolves, then the best players of the last decade or so are the best ever, in terms of absolute level. And that would be Novak in 2011/15/early16, Rafa in 2008/10/13, and Roger in 2004-07. On the other hand, I prefer relative peak because a player can only ever play in the context they played in. In that case, from what I've heard you have to look at Lew Hoad--whom I'm guessing no one saw play--as the player capable of the greatest level in tennis history.
We could also split it up by court type. The highest peak on clay is Rafa, on outdoor and slow hard courts is Novak, on grass, indoor, and fast hard courts is Roger.