Mantle, Mays, Aaron: percent above league.

Career OPS (On Base Plus Slugging):
Mickey Mantle 36.64%
Willie Mays 28.90%
Hank Aaron 28.71%

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name    BA%     OBA%    SLG%    OPS%
Mantle 16.41% 27.96% 43.93% 36.64%
Mays 14.39% 16.72% 39.25% 28.90%
Aaron 16.86% 14.37% 40.86% 28.71%

baseball-reference.com definition: a league average (non-pitcher) would have had in the same park(s).
BA: batting average
OBA: On Base Average, a.k.a. On Base Percentage
SLG: Slugging average