Players Roger lost to, by year:
2005: Rafael Nadal, Marat Safin, David Nalbandian, Richard Gasquet
2006: Rafael Nadal x4, Andy Murray
2007: Rafael Nadal x3, Guillermo Canas x2, Fernando Gonzalez, David Nalbandian, Novak Djokovic, Filippo Volandri
2008: Rafael Nadal x4, Andy Murray x2, James Blake x2, Gilles Simon x2, Radek Stepanek, Andy Roddick, Mardy Fish, Novak Djokovic
I think that says a lot about Roger's level. You can also look at his winning percentage against players other than Rafa, Novak and Andy to get a sense of his baseline level. In 2006 he was 88-0 (!) against non-Big Four players. In 2007 that dropped to 62-6 and 63-7 in 2008. Actually, interestingly enough he dropped about 1% in each year from 2007 to 2010 against non-Big 4: 91.2% in 2007, 90% in 2008, 88.9% in 2009, 87.9% in 2010, then jumped up to 92.5% in 2011 and 91.5% in 2012. 2011 was the first true "Djodal" year, with Roger a clear #3 behind the two. In 2012 he caught up a bit, and then collapsed in 2013 (81.8% against non-Big Four), and re-turned to his previous 2008-12 level in 2014 (88.2%) and 2015 (90.5%).
Roger pretty much exemplifies the idea of a "post-peak plateau." His true peak was 2004-06, with 2007 still very high but a step down, and then 2008 beginning a new baseline plateau, which fluctuated somewhat but has been pretty consistent.
2017 is interesting in that if you take Rafa out of it, his overall performance isn't that different than his plateau. He's 45-4 vs non-Big Four, or 91.8%..so it is a "good plateau year" like 2011-12, but not quite to the level of 2004-06. He has some strange losses (Haas, Donskoy) that wouldn't have happened in his peak, coupled with a loss to a young stud (Zverev) and a sometime-elite (Del Potro).