Are the New York Yankees the most winning team in Major League Baseball history?
They're the most winningest team in any sport. (As far as World Championships are concerned.) They have 27 World Series Championships.
The Yankees have the highest all-time winning percentage of any team in MLB. But several teams have more wins than they do, mainly because those teams have been around longer. As of April 3, 2010 those teams with more wins than the Yankees are the Cardinals, Pirates, Dodgers, Reds, Cubs, Giants and Braves.