Despite the obvious cycling and rowing Olympians and the World Cup winners, I think I would choose Daley Thompson, on the basis that nothing compares to being the world's best all-round athlete for such a long period and also having a down to earth character, being a role model for black urban youth, etc.
A contender for Top 5 might be W.G. Grace.
A contender for Top 5 might be W.G. Grace.