The Pittsburg Pirates 7 World Series Appearances and they have won 5. 71%

The other results for teams

Yankees- 40, won 27 67%

Giants 20, Won 8 40%

Cardinals 19, Won 11 57%

A's 14, Won 9 64%

Dodgers 18, Won 6 33%

Red Sox 12, Won 8 66%

Cubs... 10, Won 2 20%

Reds 9, Won 5 55%

Braves 9, Won 3 33%