Baseball - Babe Ruth, could have made the Hall of Fame as a pitcher as well had he stuck with it.
Basketball - Bill Russell. 13 NBA seasons, 11 NBA titles. 22 rebounds a game, could dominate a game without scoring a single point.
Football - Jim Brown. 9 seasons, led NFL in rushing 8 times. Career averages of over 100 yards per game and 5.2 yards per carry.
Hockey - Wayne Gretzky. NHL record book is dominated by #99.
Golf - Jack Nicklaus. 18 majors. Tiger's got a long way to go to catch that record.
Soccer - Pele. If Americans know about him, then he MUST be the greatest.
Boxing - Ali is the best heavyweight, Sugar Ray Robinson is the best non-heavyweight.
Tennis - Rod Laver. Two grand slams.