Looking at the numbers, there's a clear regression the further we move Fed's peak away from 2003. However, it is interesting to note that his totals vs. top ten players goes up and, coincidentally or not, his winning % against them goes down. It goes back to the chicken/egg argument: did Federer decline or did the competition get stronger? I think it's clearly both, but I'm sure some will have strong opinions either way.
And although raw totals would go down in terms of most achievements, we could, for fun and games, say Federer's peak was 2006-2010 and his total matches against top ten players would increase again but without serious damage to his percentage (2005, he went 16-2 against top 10, and in 2010, 15-6).
Looked it over, and from 2006-2010, his % against top ten is 67.3%, and the latter half of this period is when the "Big Four" really started coming into their own. He'd still have 10 GS titles to his credit in this period, as well as 3 #1 finishes from the ages of 24-29, and it's still arguably more impressive than anything Nadal did in a stretch of five years, in my opinion.
Great stuff, particularly re: the total matches played against the Top 10 as the 2000s crept along. Plenty of ways to read that data, all worth discussing.
I might go back and add record against Top 5 opponents to the Nadal entries, just to see if any differences emerge.
EDIT: Just added the Top 5 numbers. First thought is that it's interesting how Nadal played better against 1-5 than 6-10 in the 2006-09 range or so, particularly when compared to how much better he played the second half of the top 10 in later years (33-5 against those guys from 2010-13; 27-3 from 2011-13; 19-0 from 2012/13). Might be due to facing the Top 5 on preferred clay more often in early years:
2006 - 8 matches against the Top 5; 4 on clay, 1 grass, 1 outdoor hard, 2 indoor hard. 6-2 overall record - 4-0 on clay, 0-1 on grass, 1-0 on outdoor hard, 1-1 on indoor hard.
2007 - 12 matches against the Top 5; 6 on clay, 2 grass, 2 outdoor hard, 2 indoor hard. 8-4 overall record - 5-1 on clay, 1-1 on grass, 1-1 on outdoor hard, 1-1 on indoor hard.
2008 - 14 matches against the Top 5; 8 on clay, 2 grass, 4 outdoor hard. 11-3 overall record - 8-0 on clay, 2-0 on grass, 1-3 on outdoor hard.
2009 - 12 matches against the Top 5; 6 on clay, 3 outdoor hard, 3 indoor hard. 7-5 overall record - 5-1 on clay, 2-1 on outdoor hard, 0-3 on indoor hard.
2010 - 8 matches against the Top 5; 1 on clay, 1 grass, 3 outdoor hard, 3 indoor hard. 5-3 overall record - 1-0 on clay, 1-0 on grass, 1-2 on outdoor hard, 2-1 on indoor hard.
2011 - 16 matches against the Top 5; 7 on clay, 2 grass, 6 outdoor hard, 1 indoor hard. 8-8 overall record - 5-2 on clay, 1-1 on grass, 2-4 on outdoor hard, 0-1 on indoor hard.
2012 - 6 matches against the Top 5; 3 on clay, 3 on outdoor hard. 4-2 record overall - 3-0 on clay, 1-2 on outdoor hard.
2013 - 17 matches against the Top 5; 7 on clay, 7 on outdoor hard, 3 on indoor hard. 12-5 record overall - 6-1 on clay, 5-2 on outdoor hard, 1-2 on indoor hard.
The main takeaway here is that even if his peak level play is largely recognized to be in the 2007-10 range, Nadal has certainly become a more consistent player across the tour in recent years - particularly on HCs.
Through 2009, Nadal was meeting his peers in the rankings about half the time or more on clay; since then, it's generally around half the time or more on hard courts. His HC results against the Top 5 are still not great in the 2009-13 range (14-18 by my count, versus 7-10 in the 2006-10 period), but he's meeting Top 5 players a lot more frequently on HCs than he was back then, which means of course he's generally progressing further in those events.
Still maintaining clay superiority over the Top 5 in any 5 year stretch, of course.
Anyway, perhaps this data suggests favoring the 2009-13 range. Though, again, it's just hard to skip 2008, including that 10-0 result on clay/grass against Top 5 players.