クレイトン・カーショウ
This is a list of the lowest ERAs among pitchers that played their entire careers after 1920 (minimum 1,000 innings pitched). Note that four of the top seven (Clayton Kershaw, Jacob deGrom, Whitey Ford and Sandy Koufax) were primarily starting pitchers. Hoyt Wilhelm was a reliever for most of his career, while the other two are closers.