Babe Ruth
The gap between Ruth and his contemporaries remains the largest any team-sport athlete has opened. His 182.6 career WAR leads all players, and the shape of it is absurd: before becoming a full-time hitter he went 94-46 as a pitcher with a 2.28 ERA, led the AL with a 1.75 ERA in 1916, and threw nearly 30 consecutive scoreless World Series innings. Then he invented modern offense. His 54 home runs in 1920 exceeded the total of every other American League team; his 60 in 1927 out-homered each AL rival again. His career .342 average, .474 on-base percentage, and 1.164 OPS — still the all-time record — came with a slugging percentage (.690) no qualified career has matched. He faced a segregated talent pool, which the debate must acknowledge, but dominance relative to environment is the fairest cross-era test, and by that test Ruth has no peer.