Actually it does tell you how tough the competition was, but not in the way you imply. Federer played in the final of every tournament but one that year, if those active slam winners didn't play him it's because the competition was too tough for them to get there. The idea that the top players always getting deep in tournaments is a sign of depth and strength is utterly bizarre to me. It's the complete opposite actually, and I'm amazed I have to point that out
Neither parity nor a clear hierarchy behind the No. 1 speaks for tough competition on its own as they can be caused by a certain weakness at the top or a weakness below the top respectively. In my opinion both 2006 and 2015 overall weren't particularly strong years competition wise.
In 2006 Rafa was already great on clay but apart from that nobody was doing especially well with any consistency. Davydenko, Blake and Ljubicic as the players ranked 3, 4 and 5 in the Year-End-Ranking definitely doesn't indicate extremely tough competition.
In 2015 Murray was quite good from the start of the year until Toronto and Roger did very well in the second half of the year, especially from Halle until the US Open, but other than that and Wawrinka's performance at the French Open the level of the field was rather mediocre most of the time.
Also in both years the dominant player missed just one final (Roger in Cincinatti, Novak in Doha), which makes the differences really small. So which one you assess better comes down to if you value the higher number of titles, wins, better winning percentage and only two "weak" losses (against Rafa in Dubai and Murray in Cincinatti, whereas Novak on paper was expected to win every match he lost in 2015) of Roger in 2006 higher than Novak's two additional Masters titles, significantly bigger number of wins against Top 10 players and the fact that he lead every relevant H2H (while Roger was 2:4 against Rafa in 2006) in 2015.
In ranking points Novak finished 2015 with 16585 while Roger when using the current system would have been at something between 15500 and 16000 (depending on how many of the smaller tournaments he won you treat as 250 and how many as 500) at the end of 2006. But because in 2006 Masters weren't yet mandatory the way they are since the 250/500/1000 structure was introduced I don't think there is any argument that you can base calling one season objectively better than the other on.