Which sport are you talking about. Hockey, Soccer, Basketball, Baseball, Football, Etc.......
New England Patriots
The San Antonio Spurs have the best winning percentage over the past 13 years out of the four major sports in the US.
The New Zealand rugby team, the All Blacks, have an all-time 74.49% winning percentage.
49ers
NY Mets
New york yankees
alabama, lsu
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
Pittsburgh Steelers
Tampa bay rays