I'm not sure your premise is correct. The U.S. is possibly the "best: at Basketball -- loosing in the Olympics just 3 times since the sport was added and arguably this sport is played worldwide. The U.S. is probably "best" at U.S. football, but aside from the Canadian Football League and the Australian Football League (which both play by slightly differrent rules) nobody else really plays it, so it's kind of a silly example. You can argue for baseball since it was made an official Olympic sport in 1992 and is played in Japan and in parts of Latin America and Canada. Hockey: I would argue against us being at the top of that sport -- that honor certainly must go to the Canadians given Olympic history and even just glancing down the roster of NHL teams. Arguable the most international sport -- association football (or soccer for Americans) -- the U.S. has not one a single World Cup or Olympic gold in that sport and does not even have its own league (the U.S. world cup players usually plays in non-American leagues).

If you go with a wider lens, the U.S. is usually near the top in medal standings at the Olympics (but not usually the very top recently), but even this is slightly misleading since many sports (snowboarding being a good example) were invented in the U.S. so we would seem to have an advantage already.

In essence I don't think you have asked a well-posed question since world rankings for sports are rather subjective and a country being "at the top" of a sport has a lot to do with the history of that particular sport in that country and the U.S. tends to only only be more or less dominant in sports actually invented in the U.S.