2011.
Fed's level is often overlooked that year, because he didn't win a slam. But his level throughout the year was actually very high, mostly having losses to just Djokovic and Nadal.
The US Open loss was disappointing from him...but he went on a 17 match win streak after that. Obviously we already know about Djokovic's level that year.
Was tempted to say 2012, but Nadal being out after Wimbledon brings it down a bit. Fed himself also had a drop in level after Cincinnati.
2008 is also very strong, but Djokovic's Wimbledon performance brings it down a bit. Also Fed didn't really play well on hard courts that year...outside of the Open.