Since MLB went to a 162 game regular season in 1962*, only two teams have won more than 70% of their games. The New York Yankees in 1998 won 114 games (70.4%) and the Seattle Mariners won 116 (71.6%) in 2001.
Pre-season favourites for the 2016 World Series were the Chicago Cubs, and they remain so (4.8 on Betfair) with a rather impressive winning percentage to date of 72.2% (39-15).
However, it's worth mentioning that the last time a National League team had the best regular season record and went on to win the World Series was in 1986 (New York Mets). Since then, only five teams with the best regular season record (all American League) have gone on to win the World Series The Chicago Cubs of course, play in the National League.
As I wrote last season, the conclusion that the favourite–longshot bias is found in reverse in baseball doesn't seem to be the case any longer. Don't tell anyone.