I will base this on a scale of 1-10
2003: Roddick, Federer, Ferrero, Coria, and Nalbandian were in great form. Agassi was still quite good. Hewitt and Safin were in bad form. score=6
2004: Federer, Roddick, Hewitt, Safin, Henman, Caroia, were in great form. Agassi was still quite good. score=7
2005: Federer, Nadal Roddick, Hewitt, Davydenko, Safin, and Nalbandian were in great form. Agassi was still quite good, as was Coria. Score=8
2006: Federer, Nadal, Davydenko, and Blake were in great form. Roddick was in good form, Hewitt and Safin stunk. Score=5
2007: Federer, Nadal, Djokovic, Roddick, Davydenko, and Nalbandian were in great form. Murray was very good, Hewitt and Safin stunk. Score=8
2008: Federer, Nadal, Djokovic, and Murray were in great form, as was Davydenko, Roddick was good, as was Delpo. Score=8
2009: Federer, Nadal, Djokovic, Muray, Delpo, Roddick, and Soderling were in great form......Score=9
2010: Federer, Nadal, Djokovic, Murray, Soderling, Berdych were in great form, Roddick Davydenko, Nalbandian, Safin all stunk... score=7
2011: Djoker, Nadal, Murray in great form, Fed was in very good form, Berdych in Tsonga towards peak, Delpo in bad form, Roddick washed up score=6
2012: Djoker, Murray in great form, Fed and Nadal very good, Berdych, Tsonga, Delpo close to peak, Ferrer towards peak...... score=7
2013: Nadal and Djoker in great form, as is Murray, Berdych, Stan, and Delpo towards peak, Federer in terrible form due to injures. score=7
2014: Djoker, Nadal, Stan, and Murray near peak, Federer very good, Raonic, Berdych, Cilic near peak, as was Ferrer.....strong year...score=9
2015: Djoker, Murray, Stan, Berdych near their peak, Fed is very good, Tsonga and Nishi still towards peak......good year.........score=7
2016: Djoker peak for half year, Murray at peak, Fed and Nadal missed at least half of the year, Stan, Nishi, Cilic near peak...weak year....score=5
2017: Nadal and Fed were very good, Djoker and Murray injured for a good chunk, Stan near peak for 6 months, Cilic near peak..weak year...score=5
2018: Djoker near peak for half of year, Fed was good for 3 months, Nadal good for 9 months, Murray MIA, Stan terrible, Cilic very good....weak year...score=5
That's my two cents.
2014 to me was tough. Federer came back strong with his new racket. Djoker was tough, Stan became a slam winner and because a true heavyweight this year, Murray was near peak for most of the year, Raonic splashed onto the scene. This year was extremely tough. And Nobody won more than 1 slam each, which shows strength.
2005 and 2008 battle it out for the next toughest. Nadal was a monster in 2005, along with being a slam winner. Hewitt and Safin were very tough, as was Roddick(These 3 gave 28-31 year old Sampras a very tough time; Safin and Hewitt destroyed Sampras in the USO finals in back to back years). Agassi was still very tough with his great ball-striking(similar to 2015-2016 Fed).
That's my 2 cents. Once again, none of this can be proven. Some matches are swaying my decision, like Safin taking out Federer in the 2005 AO semi, or Nadal taking out Fed at the 2008 Wimby final.