Teams with at least 4 World Series appearances, the Pittsburgh Pirates have the best winning percentage. 5 World Series titles in 7 Appearances, 5 -2 for a .714 winning percentage. Teams with at least 10 World Series appearances, the New York Yankees have the best winning percentage. 27 World Series titles in 40 Appearances, 27 -13 for a .675 winning percentage.
This is best answered in opposition to your question i.e. How do teams qualify for the playoffs? The 8 division winners and 2 wild card teams from each conference. Wild card teams are determined by the best overall winning percentage from the remaining (non division winning) teams.
As far as MLB is concerned, winning percentage doesn't really enter in to whether a team makes the playoffs or not. A team makes the playoffs by winning their division or being the team with the highest winning percentage of those other teams that did not win their division. There have been teams to make the playoffs with a record as 'poor' as 83-78, as did the 2006 St. Louis Cardinals who went on to win the World Series. There have been teams with records as good as 94-68 (1982 Baltimore Orioles) that did not make the playoffs. That goes to show that a team with a winning percentage of just over .500 can make the playoffs where a team with a winning percentage of .580 can miss the playoffs.
Game 1 winners have a 367-164 advantage for a 69% chance of winning the series.
The top two teams who have won in there division the most have a playoff
currently 1. thunder 80.6 winning percentage 2.bulle 78.9 winning percentage 3. heat 77.8 winning percentage 4.spurs 69.4 winning percentage 5. pacers 64.7 winning percentage