I am a huge fan of Federer and Michael Jackson but I can accept the truth.
Wimbledon 2008: According to this New York Times' article doctors said Federer was recovered from the adverse effects of mononucleosis as soon as late February. He received medical clearance to play normally the 27th of February 2008. Reference:
http://www.nytimes.com/2008/03/07/sports/07iht-arena.3.10811374.html?pagewanted=all
Federer arrived to the Wimbledon final in July (more than 4 months after the medical clearance). Federer was moving perfectly in Wimbledon 2008, and he arrived to the final without losing any set. He was 26 and didn't look less fast than in 2007. So he was at his peak. Even the 2007 final was kinda close, Nadal had 4 break points in the 5th set. 26 years old Roger was 100% healthy and at his peak in the 2008 Wimbledon final.
Australian Open 2009: healthy Federer who was only 27 years old, peak version. 4 out of 4 GS finals for 2009 Federer. 2009 Federer arrived to the Australian Open final (lost to Nadal), won the French Open, won Wimbledon against an inspired Roddick, and arrived to the US Open final (lost to peak Del Potro).
US Open 2013: 26 years old peak and healthy Djokovic. Djokovic's 2013 could have been one of his best years if not for Nadal. If not for Nadal, Djokovic could have won 3 GS + the WTF in 2013. He still had a good year by winning Australian Open and WTF, but lost to Nadal at the French Open and US Open.