Pedro's 2000 was pretty easily the best of the past 120 years when just looking at the season he had in relation to the rest of the league that year.
Arguments can and should be made regarding number of innings pitched, games completed, teams in the league, increase of player pool, introduction of the "home run at any time" (thus pitcher can't take pitches or batters "off"), etc.
But in terms of ERA+. it's not that close.
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