Carsomyr
Legend
I wrote this above, but I'd go with 2006-10 over 2007-11. I know 2011 had an extra slam final and better results on slow hard courts, but I think 2006 was ultimately a better year - more titles on more surfaces, better w/p overall, etc. - plus Nadal protected his home surface better (right in the middle of his 81 match clay winning streak), had a slightly better fall season (to his standards), and had that great win on the fast courts of Dubai to snap Fed's 56 match HC winning streak.
2011 does have the Davis Cup win, however, and great consistency across surfaces - it's a close call.
But then there's this caveat: in 2011, Nadal was a more achieved player than in 2006, and all the nuances that entails - the aura of entering the season as the best player in the world, coming off a three-slam year, for example.
Same thing with Federer: '03-'07 or '04-'08? Arguments could be made for including either '03 and '08 in that stretch. In 2003, it's arguable he was the best player in the world, but was very underwhelming at the majors other than his Wimbledon win. In 2008, bad losses crept up and Nadal started permanently turning the tide, but his major results were far better than in 2003 and he had the 12 major aura to his credit.