Baseball- Babe Ruth (most dominant hitter of all time and throw in one of the top left handed pitchers in the league over a multi-year period with Boston)
Football- Jim Brown (arguably easily the best running back in the sport, almost every year he played during an era when the NFL was a running back oriented league).
Hockey- Gordie Howe (comtempoary Bobby Hull's slap shot was 125 mph, Gordies's WRIST shot (no wind-up) was 118 mph, best defensive great offensive player ever, and he was his own enforcer as the toughest player in the game).
Basketball- Wilt Chamberlain (this was the toughest and I could have gone a few other ways, but Wilt's scoring and rebounding were beyond compare, and he even led the league in assists one year just to make a point. While his teams often came up just short to some great champions, he was still part of two of the greatest teams of all time- 1967 in Philly and 1972 in LA.
Golf- Jack- (not just the 18 majors but the 19 seconds and while yes the overall quality depth of competition was less- he faced and often beat some of the toughest competitor's in the game including Arnie, Trevino, Player, and Watson.)
Boxing- Ali- (pre suspension he was the fastest heavyweight ever (both feet and hands) with underrated punching power (watch the Cleveland Williams fight). After a 3 year layoff he came back and retained the hand speed but changed his style and still beat 2 great champions in Frazier (2 of 3) and Foreman.