Basebatt
50s Mickey Mantle but not by much, some great players in that decade.
60s Hank Aaron by a mile.
70s Tom Seaver he sure was terrific.
80s Wade Boggs 352 batting average for the decade.
90s Roger Clemens with Greg Maddux a close second.
00s Albert Pujols and it isn't even close.
Those are my picks.
EDIT: just notice the rookie part so will have to change my picks.