Through Super Bowl XLII, that would be the Carolina Panthers at 6-3 for a winning percentage of .667 and the Baltimore Ravens at 5-3 for a winning percentage of .625. For teams that have been around a little longer than the Panthers and Ravens, it would be the New England Patriots at 21-13 for a winning percentage of .618 and the Green Bay Packers at 24-15 for a winning percentage of .615.
49ers
alabama, lsu
NY Mets
As of the 2013 season, the college football team that has the best winning percentage in Division 1 is Old Dominion with a winning percentage of .767. Old Dominion has only played 60 games in Division 1 though. Notre Dame is 2nd with a percentage of .733 Michigan is 3rd with a percentage of .732
Through the 2010 season, it's the Chicago Bears. They have a record of 704-512-42 for a winning percentage of .579.
Manchester United
Tampa bay rays
Pittsburgh Steelers
New England Patriots
Through the 2007 season, the American League team with the most wins in a regular season was the 2001 Seattle Mariners with 116. The American League team with the best winning percentage in a regular season is the 1954 Cleveland Indians who went 111-43 for a winning percentage of .721. Seattle's record in 2001 was 116-46 for a winning percentage of .716.
According to the Pro Football Hall of Fame, as of the start of the 2007 season the team with the best all time regular season winning percentage was the Miami Dolphins whose all time regular season record was 368-252-4 for a winning percentage of .593. Second was the Chicago Bears with an all time regular season record of 670-482-42 for a winning percentage of .579.