Just going back to the Kershaw/Koufax discussion from last thread, like Aub said, when adjusting for league Kershaw's peak has been superior.
Kershaw's 2013-16 ERA+:
194
197
173
230
Koufax's 1963-66 ERA+:
159
186
160
190
Koufax's 63, 65, and 66 seasons might look more amazing because of the amount of innings he threw, but that was just the era of pitching. Drysdale threw 300+ innings for three of those mid 60s seasons as well, and there were 88 pitchers through 63-66 that tossed more innings in a season than Kershaw has as a season high (236).
What Kershaw was doing this year was '99 Pedro levels of insanity.