The highest all time winning percentages team: all time record
The Yankees are first
... at a whopping .567
best post war win %age in a single season:
over a 7 game series, the best reg. season team in post war history would be expected to win .721* 7 = 5 games against the median team in MLB. the playoffs obviously have teams that are better than the hypotheitcal median team, so the best team in reg season history (post war) would be expected to win less than 5 games in a 7 game series.